pytorch/torch/distributed/fsdp/flat_param.py

import contextlib
import os
import warnings
from enum import auto, Enum
from itertools import accumulate, chain
from typing import (
    Any,
    Callable,
    Dict,
    Generator,
    Iterator,
    List,
    NamedTuple,
    no_type_check,
    Optional,
    Sequence,
    Set,
    Tuple,
    Union,
)

import torch
import torch.distributed as dist
import torch.nn as nn
import torch.nn.functional as F
from torch import Tensor
from torch.distributed._tensor import DTensor
from torch.distributed.fsdp._common_utils import (
    _set_fsdp_flattened,
    HandleTrainingState,
)
from torch.distributed.utils import _alloc_storage, _free_storage, _p_assert

from ._fsdp_extensions import _ext_post_unflatten_transform, _ext_pre_flatten_transform
from ._utils import _no_dispatch_record_stream, _same_storage

__all__ = [
    "FlatParameter",
    "FlatParamHandle",
    "FlatParamShardMetadata",
    "ParamInfo",
    "SharedParamInfo",
    "HandleShardingStrategy",
]


"""
[Note: Fully Sharded Module]
We define the "fully sharded module" to be the original ``nn.Module`` that owns
a ``FlatParamHandle``. It is the *single* module logically responsible for the
*single* unshard/reshard pair for the handle's ``FlatParameter`` for a given
forward or backward pass. The fully sharded module should be passed to the
``FlatParamHandle`` constructor.

For the wrapper code path:
- The ``FullyShardedDataParallel`` module wrapping the fully sharded module
runs the unshard/reshard on behalf of the fully sharded module by overriding
``nn.Module.forward``.
- The fully sharded module is exactly the module passed to the
``FullyShardedDataParallel`` constructor's ``module`` argument.

For the non-wrapper code path:
- Hooks registered on the fully sharded module run the unshard/reshard.
- The fully sharded module may either be the direct argument to ``fully_shard``
or a submodule chosen by the provided wrapping policy.
"""

# Environment variable toggling whether to use unsafe `setattr()` for view
# setting in `_use_sharded_views()` and `_use_unsharded_views()`
# We should use 'safe' by default since it respects method overrides, but for
# special cases such as for high CPU overhead or for intentionally bypassing
# checks in the overrides, we may use 'unsafe'.
_FSDP_USE_UNSAFE_SETATTR = "FSDP_USE_UNSAFE_SETATTR"


class ParamInfo(NamedTuple):
    """Information for an original module parameter."""

    param_name: str  # unprefixed
    module: nn.Module
    module_name: str


class SharedParamInfo(NamedTuple):
    """
    Additional information for a shared parameter.

    For each shared parameter, we designate one module and its parameter
    variable to be the primary owner, determined as the first one encountered
    in the parameter walk. These are prefixed with "prim". The primary module
    and parameter do not have their own :class:`SharedParamInfo` instance.
    """

    param_name: str  # unprefixed
    module: nn.Module
    module_name: str
    prim_param_name: str  # unprefixed
    prim_module: nn.Module
    prim_module_name: str


class FlatParamShardMetadata(NamedTuple):
    """
    This holds metadata specific to this rank's shard of the flattened
    parameter.

    Attributes:
        param_names (Tuple[str, ...]): Prefixed parameter names of this rank's
            shard of the parameters; see :class:`FlatParameter`.
        param_shapes (Tuple[torch.Size, ...]): Parameter shapes of this rank's
            shard of the parameters; see :class:`FlatParameter`.
        param_numels (Tuple[int, ...]): Parameter numels of this rank's shard
            of the parameters; see :class:`FlatParameter`.
        param_offsets (Tuple[Tuple[int, int], ...]): [start, end] offsets (in
            units of numels) giving this rank's part of each flattened
            original module parameter.
    """

    param_names: Tuple[str, ...]
    param_shapes: Tuple[torch.Size, ...]
    param_numels: Tuple[int, ...]
    param_offsets: Tuple[Tuple[int, int], ...]


# TODO (awgu): Prefix these with "Handle" for now to avoid circular imports and
# inadvertent misuses; coalesce with those in fully_sharded_data_parallel.py
# later
class HandleShardingStrategy(Enum):
    FULL_SHARD = auto()
    SHARD_GRAD_OP = auto()
    NO_SHARD = auto()
    HYBRID_SHARD = auto()
    _HYBRID_SHARD_ZERO2 = auto()


class FlatParameter(nn.Parameter):
    """
    This is the flattened parameter used by :class:`FullyShardedDataParallel`.
    It is comprised of one or more original parameters, which are flattened
    and concatenated to construct the flattened parameter.

    Under the current design, this parameter logically represents both the
    unsharded and sharded flattened parameter, and its data changes storages
    dynamically.
        - In the :class:`FullyShardedDataParallel` constructor, the parameter
        is initialized as unsharded and then sharded in-place.
        - At runtime, the parameter is lazily (re)-initialized. The sharded
        parameter data is saved in ``self._local_shard``, and a new ``Tensor``
        ``self._full_param_padded`` is created, which is the all-gather
        destination and owns the unsharded parameter storage thereafter. (See
        :meth:`FlatParamHandle.init_flat_param_attributes`.)
        - Throughout runtime, the parameter data changes storages as needed,
        e.g. to the sharded flattened parameter, reduced-precision sharded
        flattened parameter, or the unsharded flattened parameter.

    Attributes:
        _unpadded_unsharded_size (torch.Size): Unsharded flattened parameter's
            size without padding.
        _padded_unsharded_size (torch.Size): Unsharded flattened parameter's
            size with padding. This is only set for sharded strategies since
            they require padding for the all-gather.
        _sharded_size (torch.Size): Sharded flattened parameter's size with
            padding. This is also set for ``NO_SHARD``, in which case it is the
            same as the unsharded sizes. (We omit "padded" because there is no
            analogous unpadded one.)

        _param_infos (Tuple[ParamInfo, ...]): Each parameter's parameter info
            entry; see :class:`ParamInfo`.
        _numels (Tuple[int, ...]): Each parameter's numel.
        _shapes (Tuple[torch.Size, ...]): Each parameter's shape.
        _fqns (Tuple[str, ...]): The original parameters' FQNs prefixed from
            the owning handle's ``_fully_sharded_module``. The names are
            guaranteed to be unique within the subtree rooted at that module.
        _num_params (int): Number of original parameters flattened into this
            flattened parameter; this is the length of ``_param_infos``,
            ``_numels``, ``_shapes``, and ``_fqns``.
        _shared_param_infos (Tuple[SharedParamInfo, ...]): Shared parameter
            info entries; see :class:`SharedParamInfo`.
        _param_extensions (Tuple[Optional[Any], ...]): Parameter extensions
            (i.e. some per-parameter state) used to customize pre-flatten and
            post-unflatten behavior. This is experimental, and users should not
            depend on its existence in the future.
        _modules (Set[nn.Module]): Modules that contain some original parameter
            that is flattened into the ``FlatParameter``.

        _shard_param_offsets (List[Tuple[int, int])): [start, end] offsets (in
            units of numel) giving this rank's part of each flattened original
            module parameter; for any parameter ``p`` that is not sharded
            across ranks, this will be [0, ``p.numel()``-1].
        _shard_indices (Tuple[int, int]): [start, end] indices (in units of
            parameters) for this rank's shard of the original model parameters,
            where the parameters follow the order in which they were originally
            flattened; this indexes appropriately into any data structure that
            follows the flattening order (e.g. ``_param_infos``, ``_numels``,
            etc.).
        _shard_numel_padded (int): Numel padded for this rank's sharded
            flattened parameter.

        _local_shard (Tensor): Sharded flattened parameter with padding if
            using a sharded strategy. If using ``NO_SHARD``, then this is the
            unpadded unsharded flattened parameter, and there is no notion of a
            sharded flattened parameter or padded unsharded flattened
            parameter.
        _full_param_padded (Tensor): Unsharded flattened parameter with
            padding. This is not defined for ``NO_SHARD``. When using mixed
            precision for parameters, this has the low precision.
        _full_prec_full_param_padded (Tensor): Full precision unsharded
            flattened parameter with padding. This is used for unsharding
            outside of computation when using mixed precision for parameters.
            This is never defined for ``NO_SHARD``.
        _post_backward_hook_state (Tuple[AccumulateGrad, RemovableHandle]):
            Flattened parameter's :class:`AccumulateGrad` object and
            post-backward hook handle.
        _mp_shard (Tensor): Low precision sharded flattened parameter with
            padding. This is only defined when parameter mixed precision is
            enabled. For ``NO_SHARD``, this is used for computation.
        _cpu_grad (Tensor): Sharded gradient with padding stored on CPU.
            This is only defined when offloading parameters is enabled.
        _saved_grad_shard (Tensor): Sharded gradient with padding from previous
            iterations for gradient accumulation without :meth:`no_sync`.

        _params (Optional[List[nn.Parameter]]): The original parameter
            variables if ``use_orig_params=True`` and ``None`` otherwise.
        _shared_params (Optional[List[nn.Parameter]]): The original shared
            parameter variables if ``use_orig_params=True`` and ``None``
            otherwise.
        _tensors (Optional[List[Optional[Tensor]]]): This saves the ``Tensor``
            views created in the forward and tracked by autograd when
            ``use_orig_params=True`` and is ``None`` otherwise. This is to
            preserve those ``Tensor`` variables for the backward to ensure that
            the ``FlatParameter`` 's ``AccumulateGrad`` object does not change
            in which case the post-backward hook does not run. This is relevant
            for cases like reentrant activation checkpointing.
        _is_grad_none (Optional[List[bool]]): A mask over the original
            parameters' gradients indicating if it is logically ``None`` or not
            if ``use_orig_params=True`` and ``None`` otherwise. This is needed
            because only some of the parameters may have ``None`` gradient, in
            which case the ``FlatParameter`` gradient must be non-``None`` and
            must use zeros to approximate those original ``None`` gradients.
            This mask informs FSDP to set the original parameter gradients to
            ``None`` (instead of zeros) as needed.
    """

    def _init_metadata(
        self,
        param_infos: List[ParamInfo],
        numels: List[int],
        shapes: List[torch.Size],
        fqns: List[str],
        shared_param_infos: List[SharedParamInfo],
        param_extensions: List[Any],
        params: Optional[List[nn.Parameter]],
        shared_params: Optional[List[nn.Parameter]],
    ) -> None:
        """
        Initializes attributes holding metadata about the original parameters
        comprising the flattened parameter.

        We expose this method separate from the constructor to keep the
        constructor only responsible for the flattened parameter's tensor data.
        This method should only be called once per model, while the constructor
        may be called multiple times, e.g. when reloading from a checkpoint, in
        which case only the tensor data needs to be passed to the constructor.
        Since :meth:`load_state_dict` is implemented via :meth:`copy_`, the
        metadata is correctly assumed to be unchanged.

        Args:
            See the Attributes in the class docstring.
        """
        assert len(param_infos) == len(numels)
        assert len(param_infos) == len(shapes)
        assert len(param_infos) == len(fqns)
        assert len(param_infos) == len(param_extensions)
        self._num_params = len(param_infos)
        self._param_infos = tuple(param_infos)
        self._numels = tuple(numels)
        self._shapes = tuple(shapes)
        self._fqns = tuple(fqns)
        self._shared_param_infos = tuple(shared_param_infos)
        self._param_extensions = tuple(param_extensions)
        self._modules = {pi.module for pi in self._param_infos}.union(
            {spi.module for spi in self._shared_param_infos}
        )
        assert (params is None) == (shared_params is None)
        if params is not None:
            assert shared_params is not None and len(shared_params) == len(
                shared_param_infos
            )
            self._params: Optional[List[nn.Parameter]] = params
            self._shared_params: Optional[List[nn.Parameter]] = shared_params
            # Mark the original parameters to avoid flattening them into
            # another `FlatParameter` during recursive construction
            for param in chain(self._params, self._shared_params):
                _set_fsdp_flattened(param)
            self._is_grad_none: Optional[List[bool]] = [
                False for _ in range(len(params))
            ]
            self._tensors: Optional[List[Optional[Tensor]]] = [
                None for _ in range(len(self._params))
            ]
        else:
            self._params = None
            self._shared_params = None
            self._is_grad_none = None
            self._tensors = None
        self._unpadded_unsharded_size = self.size()
        _set_fsdp_flattened(self)
        # Tracks whether the `FlatParameter`'s post-backward hook has been
        # called to modify the behavior of the post-backward callback
        self._post_backward_called = False


class FlatParamHandle:
    """
    This handle manages a flattened parameter (:class:`FlatParameter`). This
    includes sharding and view management.

    Args:
        params (Sequence[nn.Parameter]): The parameters to use for the
            flattened parameter.
        fully_sharded_module (nn.Module): See [Note: Fully Sharded Module].
        device (torch.device): The compute and communication device, which
            should be a non-CPU device. We refer to it as the compute device.
        sharding_strategy (ShardingStrategy): Sharding strategy to apply to
            this handle's ``FlatParameter``.
        offload_params (bool): Whether to offload the handle's
            ``FlatParameter`` to CPU.
        mp_param_dtype (Optional[torch.dtype]): Parameter mixed precision
            setting passed to the FSDP constructor.
        mp_reduce_dtype (Optional[torch.dtype]): Gradient reduction mixed
            precision setting passed to the FSDP constructor.
        keep_low_precision_grads (bool): Whether to keep gradients in low
            precision.
        use_orig_params (bool): If ``True``, then FSDP preserves the original
            parameter variables and returns them from ``named_parameters()``
            (e.g. to support different optimizer hyperparameters within one
            :class:`FlatParameter`). If ``False``, then FSDP reconstructs the
            parameter every iteration and returns the :class:`FlatParameter` s
            from ``named_parameters()``.
    """

    ##################
    # INITIALIZATION #
    ##################
    def __init__(
        self,
        params: Sequence[nn.Parameter],
        fully_sharded_module: nn.Module,
        device: torch.device,
        sharding_strategy: HandleShardingStrategy,
        offload_params: bool,
        mp_param_dtype: Optional[torch.dtype],
        mp_reduce_dtype: Optional[torch.dtype],
        keep_low_precision_grads: bool,
        process_group: dist.ProcessGroup,
        use_orig_params: bool,
    ):
        super().__init__()
        use_unsafe_setattr = os.environ.get(_FSDP_USE_UNSAFE_SETATTR, "") == "1"
        self._setattr_tensor: Callable[[nn.Module, str, Tensor], None]
        self._setattr_param: Callable[[nn.Module, str, nn.Parameter], None]
        if use_unsafe_setattr:
            self._setattr_tensor = _unsafe_setattr_tensor
            self._setattr_param = _unsafe_setattr_param
        else:
            self._setattr_tensor = _safe_setattr_tensor_or_param
            self._setattr_param = _safe_setattr_tensor_or_param
        self.device = device
        self.process_group = process_group
        self.rank = process_group.rank()
        self.world_size = process_group.size()
        self._sharding_strategy = sharding_strategy
        self._offload_params = offload_params
        self._use_orig_params = use_orig_params
        self._keep_low_precision_grads = keep_low_precision_grads
        self._training_state = HandleTrainingState.IDLE
        self._debug_level = dist.get_debug_level()
        self._fully_sharded_module = fully_sharded_module
        self._init_flat_param(params, fully_sharded_module, use_orig_params)
        self._orig_param_dtype = self.flat_param.dtype
        self._use_unsharded_views(as_params=False)
        self._init_param_reduce_dtypes(mp_param_dtype, mp_reduce_dtype)

    def _init_flat_param(
        self,
        params: Sequence[Optional[nn.Parameter]],
        module: nn.Module,
        use_orig_params: bool,
    ) -> None:
        """
        Initializes the flattened parameter ``self.flat_param`` by flattening
        the parameters in ``params`` into a single :class:`FlatParameter` and
        saves relevant metadata. Shared parameters are only included in the
        flattened parameter once.

        This checks that all comprising parameters have the same dtype and
        ``requires_grad`` and does not support nested construction of
        :class:`FlatParameter` s.

        Args:
            See the Args in the class docstring.
        """
        params_set = set(params)
        params_set.discard(None)
        if len(params_set) == 0:
            raise ValueError(
                "Cannot initialize a `FlatParameter` from an empty parameter list"
            )
        param_infos: List[ParamInfo] = []
        numels: List[int] = []
        shapes: List[torch.Size] = []
        fqns: List[str] = []
        shared_param_infos: List[SharedParamInfo] = []
        shared_param_memo: Dict[nn.Parameter, Tuple[nn.Module, str, str]] = {}
        params_to_flatten: List[Union[torch.Tensor, nn.Parameter]] = []
        shared_params: List[Union[torch.Tensor, nn.Parameter]] = []
        param_extensions: List[Any] = []
        dtype: Optional[torch.dtype] = None
        requires_grad: Optional[bool] = None
        for submodule_name, submodule in module.named_modules():
            for param_name, param in submodule.named_parameters(recurse=False):
                if param not in params_set:
                    continue
                if param in shared_param_memo:  # shared reference
                    prim_module, prim_module_name, prim_param_name = shared_param_memo[
                        param
                    ]
                    shared_params.append(param)
                    shared_param_infos.append(
                        SharedParamInfo(
                            param_name,
                            submodule,
                            submodule_name,
                            prim_param_name,
                            prim_module,
                            prim_module_name,
                        )
                    )
                else:
                    if type(param) is FlatParameter:
                        raise ValueError("`FlatParameter` does not support nesting")
                    if dtype is not None and param.dtype != dtype:
                        raise ValueError(
                            "`FlatParameter` requires uniform dtype but got "
                            f"{dtype} and {param.dtype}"
                        )
                    if dtype is None and not param.is_floating_point():
                        raise ValueError("Integer parameters are unsupported")
                    if (
                        requires_grad is not None
                        and param.requires_grad != requires_grad
                    ):
                        raise ValueError(
                            "`FlatParameter` requires uniform `requires_grad`"
                        )
                    param, extension = _ext_pre_flatten_transform(param)
                    param_extensions.append(extension)
                    dtype = param.dtype
                    requires_grad = param.requires_grad
                    shared_param_memo[param] = (submodule, submodule_name, param_name)
                    params_to_flatten.append(param)
                    param_infos.append(ParamInfo(param_name, submodule, submodule_name))
                    numels.append(param.numel())
                    shapes.append(param.shape)
                    fqn = (
                        submodule_name + "." + param_name
                        if submodule_name
                        else param_name
                    )
                    fqns.append(fqn)
        assert requires_grad is not None, (
            "Passed-in `params` were not found in the module tree\n"
            f"params: {params}\nmodule: {module}"
        )
        self.flat_param = FlatParamHandle.flatten_params(
            params_to_flatten, requires_grad
        )
        # For `use_orig_params=True`, ensure that the logical parameters are
        # `nn.Parameter`s (and not plain `torch.Tensor`)

        def convert_to_params(
            tensors: List[Union[torch.Tensor, nn.Parameter]]
        ) -> List[nn.Parameter]:
            return [
                t if isinstance(t, nn.Parameter) else nn.Parameter(t) for t in tensors
            ]

        self.flat_param._init_metadata(
            param_infos,
            numels,
            shapes,
            fqns,
            shared_param_infos,
            param_extensions,
            convert_to_params(params_to_flatten) if use_orig_params else None,
            convert_to_params(shared_params) if use_orig_params else None,
        )

    @staticmethod
    def flatten_params(
        params: Sequence[torch.Tensor],
        requires_grad: bool,
    ) -> FlatParameter:
        """
        Flattens the parameters in ``params`` into a single
        :class:`FlatParameter`. This should be the only way used to construct
        :class:`FlatParameter` s.

        We expose this factory method for checkpointing (e.g. sharded state
        dict). The flattened parameter's metadata should only be initialized
        once (see :meth:`_init_metadata`), but its tensor data may be reloaded.
        """
        with torch.no_grad():
            flat_params = [
                p.detach().reshape(-1) if isinstance(p, nn.Parameter) else p.reshape(-1)
                for p in params
            ]
            flat_param_data = torch.cat(flat_params, dim=0)
        flat_param = FlatParameter(flat_param_data, requires_grad=requires_grad)
        return flat_param

    def _init_param_reduce_dtypes(
        self,
        mp_param_dtype: Optional[torch.dtype],
        mp_reduce_dtype: Optional[torch.dtype],
    ) -> None:
        """
        Precondition: ``self.flat_param`` is set via :meth:`_init_flat_param`.
        This ensures that this handle's parameters have a single dtype.

        Postcondition: This sets ``self._fwd_bwd_param_dtype`` and
        ``self._reduce_dtype``. If ``mp_param_dtype`` or ``mp_reduce_dtype``
        is ``None``, then we assume the original parameter dtype. One special
        case is if ``mp_param_dtype`` is not ``None`` and ``mp_reduce_dtype``
        is ``None``, in which case we assume the gradient reduction dtype
        matches the forward/backward parameter dtype.
        """
        # Save whether these dtypes were specified so that we permit the
        # parameter dtype to change up until the lazy initialization
        self._low_prec_param_dtype_specified = mp_param_dtype is not None
        self._low_prec_reduce_dtype_specified = mp_reduce_dtype is not None
        if (
            self._low_prec_param_dtype_specified
            and not self._low_prec_reduce_dtype_specified
        ):
            # Special case: infer gradient reduction mixed precision
            self._fwd_bwd_param_dtype = mp_param_dtype
            self._reduce_dtype = self._fwd_bwd_param_dtype
        else:
            self._fwd_bwd_param_dtype = mp_param_dtype or self._orig_param_dtype
            self._reduce_dtype = mp_reduce_dtype or self._orig_param_dtype
        assert self._fwd_bwd_param_dtype is not None
        assert self._reduce_dtype is not None

    ###################################
    # SHARD INITIALIZATION & METADATA #
    ###################################
    @torch.no_grad()
    def shard(self):
        """
        Shards the handle's ``FlatParameter``. In terms of memory, this
        allocates new memory for the sharded flattened parameter and frees the
        unsharded flattened parameter's storage.

        Postcondition: ``self.flat_param`` is the sharded flattened parameter.
        Shard metadata attributes are set for all sharding strategies.
        ``process_group``, ``rank``, and ``world_size`` attributes are set if
        using a sharded strategy.
        """
        flat_param = self.flat_param
        if not self.uses_sharded_strategy:
            self._init_shard_metadata(0, 0, flat_param.numel() - 1)
        else:
            _p_assert(
                flat_param.storage_offset() == 0,
                "The `FlatParameter` is not the sole occupant of its storage",
            )
            orig_storage = flat_param._typed_storage()
            sharded_flat_param, numel_padded = FlatParamHandle._get_shard(
                flat_param, self.rank, self.world_size
            )
            flat_param.set_(sharded_flat_param)  # type: ignore[call-overload]
            start = sharded_flat_param.numel() * self.rank
            end = sharded_flat_param.numel() * (self.rank + 1) - 1  # inclusive
            self._init_shard_metadata(numel_padded, start, end)
            if orig_storage._size() > 0:
                orig_storage._resize_(0)
        if self._use_orig_params:
            self._use_sharded_views()

    def _init_shard_metadata(
        self,
        numel_padded: int,
        start: int,
        end: int,
    ) -> None:
        """
        Initializes shard-related metadata for this rank's shard of the
        flattened parameter: ``_sharded_size``, ``_shard_param_offsets``,
        ``_shard_indices``, and ``_shard_numel_padded``.

        Args:
            numel_padded (int): Numel padded for this rank's sharded flattened
                parameter.
            start (int): Start index in the sharded flattened parameter
                assigned to this rank.
            end (int): End index (inclusive) in the sharded flattened parameter
                assigned to this rank. If this exceeds the sharded flattened
                parameter's numel, then it is truncated.

        Precondition: ``self.flat_param`` 's data is the sharded flattened
        parameter.
        """
        self.flat_param._sharded_size = self.flat_param.size()  # type: ignore[attr-defined]
        sharded_flat_param_numel = self.flat_param.numel()  # includes `numel_padded`
        _p_assert(start >= 0 and start <= end, f"start: {start} end: {end}")
        _p_assert(
            numel_padded <= sharded_flat_param_numel,
            f"numel_padded: {numel_padded} "
            f"sharded_flat_param_numel: {sharded_flat_param_numel}",
        )
        (
            self.flat_param._shard_param_offsets,  # type: ignore[attr-defined]
            self.flat_param._shard_indices,  # type: ignore[attr-defined]
        ) = self._get_shard_metadata(start, end)
        self.flat_param._shard_numel_padded = numel_padded  # type: ignore[attr-defined]

    def _get_shard_metadata(
        self,
        start: int,
        end: int,
    ) -> Tuple[Tuple[Tuple[int, int], ...], Tuple[int, int]]:
        """
        Computes the shard metadata based on ``start`` and ``end``, which give
        the closed interval of the unsharded flattened parameter specifying the
        shard.

        Args:
            start (int): Start index (in units of numel) of this rank's shard
                of the flattened parameter.
            end (int): End index (in units of numel and inclusive) of this
                rank's shard of the flattened parameter.

        Return:
            Tuple[Tuple[Tuple[int, int], ...], Tuple[int, int]]: See
            ``_shard_param_offsets`` and ``_shard_indices`` in
            :class:`FlatParameter` 's docstring.
        """
        flat_param_offsets = self._get_flat_param_offsets()
        # Indices of the original parameters in this rank's sharded flattened
        # parameter
        shard_param_indices_range = []  # elements will be consecutive
        # [start, end] offsets giving this rank's part of the flattened
        # original module parameter (which will be [0, `p.numel()`-1] for any
        # parameter that is not sharded across ranks)
        shard_param_offsets = []
        for i, (param_start, param_end) in enumerate(flat_param_offsets):
            if start > param_end or end < param_start:
                continue
            if start <= param_start:
                intra_param_start = 0
            else:
                intra_param_start = start - param_start
            intra_param_end = min(param_end, end) - param_start
            shard_param_indices_range.append(i)
            shard_param_offsets.append(
                (intra_param_start, intra_param_end)
            )  # both inclusive
        if len(shard_param_indices_range) == 0:
            shard_param_indices = (0, 0)
            assert len(shard_param_offsets) == 0
        else:
            shard_param_indices = (
                shard_param_indices_range[0],
                shard_param_indices_range[-1],
            )
            assert (
                len(shard_param_offsets)
                == shard_param_indices[-1] - shard_param_indices[0] + 1
            )
        return tuple(shard_param_offsets), shard_param_indices

    @staticmethod
    def _get_unpadded_shard(
        tensor: Tensor,
        rank: int,
        world_size: int,
    ) -> Tuple[Tensor, int]:
        """
        Returns the shard of ``tensor`` without any padding for the given
        ``rank`` and ``world_size`` and the numel to pad for that shard.

        If ``tensor`` is already flattened or may be viewed in the flattened
        shape (which is true in the expected usage), then this method does not
        allocate any new tensor memory.
        """
        chunks = torch.flatten(tensor).chunk(world_size)
        if len(chunks) < (rank + 1):
            # This rank gets an empty chunk fully padded with zeros since there
            # are not enough chunks across ranks
            chunk = chunks[0].new_empty(0)
        else:
            chunk = chunks[rank]
        numel_to_pad = chunks[0].numel() - chunk.numel()
        assert (
            numel_to_pad >= 0
        ), "Chunk's size should be at most the first chunk's size"
        return chunk, numel_to_pad

    @staticmethod
    def _get_shard(
        tensor: Tensor,
        rank: int,
        world_size: int,
    ) -> Tuple[Tensor, int]:
        """
        Returns the shard of ``tensor`` with padding for the given ``rank`` and
        ``world_size`` and the numel padded for that shard.

        This method allocates new memory (via :meth:`clone`) since the
        unsharded ``tensor`` may be deallocated after this method returns.
        """
        chunk, numel_to_pad = FlatParamHandle._get_unpadded_shard(
            tensor, rank, world_size
        )
        shard = chunk.clone()
        if numel_to_pad > 0:
            shard = F.pad(shard, [0, numel_to_pad])
        return shard, numel_to_pad

    @staticmethod
    def _get_sharded_size(tensor: Tensor, rank: int, world_size: int) -> torch.Size:
        """
        Returns the shape of ``tensor`` after sharding including padding. This
        requires ``tensor`` to have 1D shape and ensures that the returned
        shape is 1D.
        """
        assert len(tensor.shape) == 1, f"{tensor.shape}"
        unpadded_sharded_tensor, numel_to_pad = FlatParamHandle._get_unpadded_shard(
            tensor, rank, world_size
        )
        unpadded_sharded_size = unpadded_sharded_tensor.size()
        assert len(unpadded_sharded_size) == 1, f"{unpadded_sharded_size}"
        return torch.Size([unpadded_sharded_size[0] + numel_to_pad])

    def _get_flat_param_offsets(self) -> List[Tuple[int, int]]:
        """Returns [start, end] offsets of each original parameter's flattened
        data in the unsharded flattened parameter (without padding)."""
        cumulative_sum = list(accumulate(self.flat_param._numels))
        starts = [0] + cumulative_sum[:-1]
        ends = [end - 1 for end in cumulative_sum]  # inclusive
        param_offsets = list(zip(starts, ends))
        return param_offsets

    def shard_metadata(
        self,
    ) -> FlatParamShardMetadata:
        """Returns shard-related metadata specific to this rank's shard of the
        flattened parameter."""
        assert hasattr(self.flat_param, "_shard_indices") and hasattr(
            self.flat_param, "_shard_param_offsets"
        ), "Shard metadata has not been initialized"
        shard_param_start_index = self.flat_param._shard_indices[0]  # type: ignore[attr-defined]
        shard_param_end_index = self.flat_param._shard_indices[1]  # type: ignore[attr-defined]
        sl = (
            slice(shard_param_start_index, shard_param_end_index + 1)
            if shard_param_start_index <= shard_param_end_index
            else slice(0, 0)
        )
        return FlatParamShardMetadata(
            self.flat_param._fqns[sl],
            self.flat_param._shapes[sl],
            self.flat_param._numels[sl],
            self.flat_param._shard_param_offsets[:],  # type: ignore[attr-defined]
        )

    @no_type_check
    @torch.no_grad()
    def init_flat_param_attributes(self) -> None:
        """
        This initializes some attributes on the handle's ``FlatParameter``.
        This should be called during lazy initialization since it requires the
        parameter to be on the compute device if not offloading to CPU and we
        want to give users the chance to move the parameter appropriately after
        the FSDP constructor.

        For each tensor attribute on the ``FlatParameter``, see the unshard and
        reshard methods in this class for the allocation and free pattern.
        """
        flat_param = self.flat_param
        if flat_param.dtype != self._orig_param_dtype:
            # Entering this branch means that the user changed the parameter
            # dtype after FSDP initialization, in which case we may need to
            # refresh some saved dtype attributes (dtypes specified as a part
            # of mixed precision take precedence).
            if not self._low_prec_param_dtype_specified:
                self._fwd_bwd_param_dtype = flat_param.dtype
            # For `reduce_dtype`, require `param_dtype` was not specified since
            # then we infer the `reduce_dtype` from the specified `param_dtype`
            if (
                not self._low_prec_reduce_dtype_specified
                and not self._low_prec_param_dtype_specified
            ):
                self._reduce_dtype = flat_param.dtype
            self._orig_param_dtype = flat_param.dtype
        cpu_device = torch.device("cpu")
        if self._offload_params:
            _p_assert(
                flat_param.device == cpu_device,
                f"Expects the `FlatParameter` to be on CPU when parameter CPU "
                f"offloading is enabled, not {flat_param.device}",
            )
        else:
            self._check_on_compute_device(self.flat_param)
        flat_param._local_shard = flat_param.data
        if self._offload_params:
            # Pin the memory for faster H2D transfer
            flat_param._local_shard = flat_param._local_shard.pin_memory()
            # Pre-allocate the sharded gradient on CPU to enable non-blocking
            # D2H transfer during the backward pass
            flat_param._cpu_grad = torch.zeros_like(
                flat_param._local_shard, device=cpu_device
            ).pin_memory()
        if self._uses_param_mixed_precision:
            # For parameter mixed precision, we maintain a low precision
            # sharded tensor on the compute device to be all-gathered (for
            # sharded strategies) or directly used (for `NO_SHARD`) for
            # computation.
            flat_param._mp_shard = torch.zeros_like(
                flat_param._local_shard,
                device=self.device,
                dtype=self._fwd_bwd_param_dtype,
            )
            _free_storage(flat_param._mp_shard)
        if self.uses_sharded_strategy:
            # We maintain a padded unsharded tensor that serves as the
            # all-gather destination and owns the original parameter storages.
            unsharded_param_dtype = (
                self._fwd_bwd_param_dtype
                if self._uses_param_mixed_precision
                else flat_param.dtype
            )  # use low precision if parameter mixed precision is enabled
            padded_unsharded_numel = flat_param.numel() * self.world_size
            flat_param._full_param_padded = torch.zeros(
                padded_unsharded_numel,
                device=self.device,
                dtype=unsharded_param_dtype,
            )
            flat_param._padded_unsharded_size = flat_param._full_param_padded.size()
            _free_storage(flat_param._full_param_padded)

            if self._uses_param_mixed_precision:
                # For parameter mixed precision, we maintain a full precision
                # padded unsharded tensor for when we force full precision.
                flat_param._full_prec_full_param_padded = torch.zeros(
                    padded_unsharded_numel,
                    device=self.device,
                    dtype=flat_param.dtype,  # full precision
                )
                _free_storage(flat_param._full_prec_full_param_padded)

    ###################
    # UNSHARD/RESHARD #
    ###################
    def pre_unshard(self) -> bool:
        """
        Returns: ``False`` if this is a no-op and ``True`` otherwise.

        Postcondition: ``self.flat_param`` 's data is on the device for
        communication and is what should be all-gathered. This means that it
        matches the dtype of the expected unsharded parameter.
        """
        ret = False
        if self._use_orig_params:
            ret = self._writeback_orig_params()
        if (
            self.uses_sharded_strategy
            and not self._offload_params
            and not self.needs_unshard()
        ):
            pass  # no-op
        elif self._uses_param_mixed_precision and not self._force_full_precision:
            self._use_low_precision_shard()
            ret = True
        elif self._offload_params and self.flat_param.device != self.device:
            # NOTE: This creates a new tensor distinct from any attributes.
            self.flat_param_to(self.device, non_blocking=True)
            ret = True
        self._check_on_compute_device(self.flat_param)
        return ret

    def _use_low_precision_shard(self):
        """
        Allocates the low precision shard directly on the compute device and
        switches to using the low precision sharded flattened parameter.
        """
        self._check_low_precision_shard()
        flat_param = self.flat_param
        _alloc_storage(
            flat_param._mp_shard, flat_param._local_shard.size()  # type: ignore[attr-defined]
        )
        # `copy_()` implicitly casts to the low precision
        flat_param._mp_shard.copy_(  # type: ignore[attr-defined]
            flat_param._local_shard.to(  # type: ignore[attr-defined]
                self.device, non_blocking=True
            )
        )
        # Invariant: `_mp_shard` is always on the compute device.
        flat_param.data = flat_param._mp_shard  # type: ignore[attr-defined]

    def unshard(self):
        """
        Runs the unshard logic. This includes all-gathering the flattened
        parameter and switching to using the unsharded flattened parameter. If
        the handle does not need unsharding, then this only switches to using
        the unsharded flattened parameter. For ``NO_SHARD``, this is a no-op.

        If FSDP is in :meth:`summon_full_params` and the handle uses parameter
        mixed precision, then the parameter is forced to full precision.
        """
        if not self.needs_unshard():
            # Even when not needing an unshard, we should switch to using
            # the unsharded flattened parameter
            unsharded_flat_param = (
                self._get_padded_unsharded_flat_param()
                if self.uses_sharded_strategy
                else self.flat_param
            )
            self._use_unsharded_flat_param(unsharded_flat_param)
            return
        unsharded_flat_param = self._alloc_padded_unsharded_flat_param()
        padded_unsharded_flat_param = self._all_gather_flat_param(unsharded_flat_param)
        self._use_unsharded_flat_param(padded_unsharded_flat_param)

    def needs_unshard(self) -> bool:
        """Returns if the handle's flattened parameter needs to be unsharded."""
        if not self.uses_sharded_strategy:
            return False
        unsharded_flat_param = self._get_padded_unsharded_flat_param()
        already_unsharded = (
            unsharded_flat_param._typed_storage()._size()
            == unsharded_flat_param.numel()
        )
        return not already_unsharded

    def _alloc_padded_unsharded_flat_param(self):
        """
        Allocates the *padded* unsharded flattened parameter. The unpadded
        unsharded flattened parameter is always a view into the padded one.
        This padded parameter is saved to a different attribute on the
        ``FlatParameter`` depending on if we force full precision.
        """
        self._check_sharded_strategy()
        flat_param = self.flat_param
        unsharded_flat_param = self._get_padded_unsharded_flat_param()
        self._check_storage_freed(unsharded_flat_param)
        _alloc_storage(unsharded_flat_param, flat_param._padded_unsharded_size)  # type: ignore[attr-defined]
        return unsharded_flat_param

    def _get_padded_unsharded_flat_param(self) -> torch.Tensor:
        """
        Returns a reference to the padded unsharded flattened parameter
        depending on the calling context. This should only be called if using a
        sharded strategy.
        """
        self._check_sharded_strategy()
        flat_param = self.flat_param
        if self._force_full_precision:
            # When parameter mixed precision is enabled, we use a different
            # tensor as the all-gather destination to preserve the invariant
            # that  `_full_param_padded` is in the low precision
            unsharded_flat_param = flat_param._full_prec_full_param_padded  # type: ignore[attr-defined]
            _p_assert(
                unsharded_flat_param.dtype != self._fwd_bwd_param_dtype,
                f"Expects full precision but got {self._fwd_bwd_param_dtype}",
            )
        else:
            unsharded_flat_param = flat_param._full_param_padded  # type: ignore[attr-defined]
        return unsharded_flat_param

    def _all_gather_flat_param(
        self,
        padded_unsharded_flat_param: Tensor,
    ) -> Tensor:
        """
        All-gathers the handle's flattened parameter to the destination
        ``padded_unsharded_flat_param``, and switches to using the all-gathered
        tensor.
        """
        _p_assert(
            hasattr(self, "process_group") and hasattr(self, "world_size"),
            "Expects a process group and world size to have been set via `shard()`",
        )
        sharded_flat_param = self.flat_param.data
        expected_numel = sharded_flat_param.numel() * self.world_size
        _p_assert(
            padded_unsharded_flat_param.numel() == expected_numel,
            f"Expects {expected_numel} numel but got {padded_unsharded_flat_param.numel()}",
        )
        dist.all_gather_into_tensor(
            padded_unsharded_flat_param,
            sharded_flat_param,
            self.process_group,
        )
        return padded_unsharded_flat_param

    def _use_unsharded_flat_param(
        self,
        padded_unsharded_flat_param: torch.Tensor,
    ) -> None:
        """
        Switches to using the *unpadded* unsharded flattened parameter, which
        is a view into the *padded* unsharded flattened parameter.
        """
        unsharded_size = self.flat_param._unpadded_unsharded_size
        self.flat_param.data = padded_unsharded_flat_param[
            : unsharded_size.numel()
        ].view(
            unsharded_size
        )  # this `.view()` is not autograd visible
        in_forward = self._training_state == HandleTrainingState.FORWARD
        in_pre_backward = self._training_state == HandleTrainingState.BACKWARD_PRE
        if self._use_orig_params:
            # We use `Tensor` views in the forward so that they are tracked by
            # autograd. We use them in the pre-backward as well to support
            # reentrant activation checkpointing, which needs the views to be
            # tracked by autograd in the backward pass's recomputed forward.
            self._use_unsharded_views(
                as_params=(not in_forward and not in_pre_backward)
            )
        elif in_forward:
            self._use_unsharded_views(as_params=False)

    def post_unshard(self):
        """
        Runs the post-unshard logic. This includes freeing the low precision
        shard if needed.
        """
        if self._uses_param_mixed_precision and self.uses_sharded_strategy:
            self._free_low_precision_sharded_param()
        self._check_on_compute_device(self.flat_param)

    def _free_low_precision_sharded_param(self):
        """Frees the low precision sharded flattened parameter."""
        self._check_low_precision_shard()
        # `_mp_shard` is allocated in the pre-unshard stream, consumed in the
        # unshard stream for sharded strategies, and consumed in both the
        # unshard and default streams for `NO_SHARD`. For sharded strategies,
        # the current stream here is the unshard stream, and for `NO_SHARD`,
        # it is the default stream. For `NO_SHARD`, only recording for the
        # default stream suffices since the default stream waits for the
        # unshard stream.
        _no_dispatch_record_stream(
            self.flat_param._mp_shard, torch.cuda.current_stream()  # type: ignore[attr-defined]
        )
        _free_storage(self.flat_param._mp_shard)  # type: ignore[attr-defined]

    @torch.no_grad()
    def unshard_grad(self):
        """
        Unshards the handle's ``FlatParameter`` 's gradient. If all ranks have
        ``None`` gradient, then all original parameters will as well. This
        method performs an all-reduce and an all-gather. The additional
        all-reduce is tolerable since this method is not meant to be used on
        the computation critical path.

        Postcondition: ``_saved_grad_shard`` is defined and contains the value
        to set ``flat_param.grad`` after gradients are resharded.
        """
        if not self.uses_sharded_strategy:
            self._use_unsharded_grad_views()
            return
        flat_param = self.flat_param
        self._check_unsharded(flat_param)

        # Check if all ranks have a `None` gradient
        num_grad_none = torch.zeros(1, dtype=torch.int32, device=self.device)
        num_grad_none[0] = flat_param.grad is None
        dist.all_reduce(num_grad_none, group=self.process_group)
        if num_grad_none[0] == self.world_size:
            flat_param._saved_grad_shard = None  # type: ignore[attr-defined]
            self._use_unsharded_grad_views()
            return

        padded_unsharded_grad = torch.empty(
            flat_param._padded_unsharded_size,  # type: ignore[attr-defined]
            device=self.device,
        )
        if flat_param.grad is None:
            # In the case that only some ranks have `None` gradient, we use
            # zeros to approximate as a best effort attempt
            if self._debug_level == dist.DebugLevel.DETAIL:
                warnings.warn(
                    f"[Rank {self.rank}] Only some but not all ranks have a "
                    "`None` `FlatParameter` gradient, so FSDP is using zeros to "
                    "approximate those ranks' sharded gradients being `None`"
                )
            flat_param._saved_grad_shard = None  # type: ignore[attr-defined]
            sharded_grad = torch.zeros(flat_param._sharded_size, device=self.device)  # type: ignore[attr-defined]
        else:
            self._check_sharded(flat_param.grad)
            flat_param._saved_grad_shard = flat_param.grad  # type: ignore[attr-defined]
            sharded_grad = flat_param._saved_grad_shard  # type: ignore[attr-defined]
        dist.all_gather_into_tensor(
            padded_unsharded_grad, sharded_grad, self.process_group
        )
        unsharded_size = self.flat_param._unpadded_unsharded_size
        flat_param.grad = padded_unsharded_grad[: unsharded_size.numel()].view(
            unsharded_size
        )
        self._use_unsharded_grad_views()

    def reshard_grad(self):
        if self._use_orig_params:
            self._use_sharded_grad_views()
        if not self.uses_sharded_strategy:
            return
        self.flat_param.grad = self.flat_param._saved_grad_shard  # type: ignore[attr-defined]
        delattr(self.flat_param, "_saved_grad_shard")

    def prepare_gradient_for_backward(self):
        """
        Prepares the gradient for the backward computation by saving and
        clearing any existing sharded gradient in ``.grad`` to enable computing
        a new unsharded gradient.
        """
        _p_assert(
            self._training_state
            in (HandleTrainingState.BACKWARD_PRE, HandleTrainingState.IDLE),
            "Expects to be in `BACKWARD_PRE` or `IDLE` (if prefetching)",
        )
        flat_param = self.flat_param
        if flat_param.grad is not None and (
            flat_param.grad.size() != flat_param._unpadded_unsharded_size
            or flat_param.grad.device != flat_param.device  # grad on CPU
        ):
            self._check_on_compute_device(self.flat_param)
            grad_offloaded = flat_param.grad.device != self.device
            _p_assert(
                not grad_offloaded or self._offload_params,
                f"Expects the sharded gradient to be on {self.device} "
                f"but got {flat_param.grad.device}",
            )
            prev_iter_synced_gradients = (
                flat_param.grad.size()
                == flat_param._local_shard.size()  # type: ignore[attr-defined]
            )
            if prev_iter_synced_gradients:
                # TODO (awgu): Gradient accumulation outside `no_sync()`
                # does not work with CPU offloading. The issue should be
                # that, in the post-backward hook, we cannot do an addition
                # between a CPU tensor (the existing sharded gradient) and
                # a GPU tensor (the new sharded gradient).
                if not grad_offloaded:
                    flat_param._saved_grad_shard = flat_param.grad.data  # type: ignore[attr-defined]
                    sharded_grad = flat_param._saved_grad_shard  # type: ignore[attr-defined]
                else:
                    _p_assert(
                        hasattr(flat_param, "_cpu_grad"),
                        "`_cpu_grad` should be defined if the gradient is on CPU",
                    )
                    sharded_grad = flat_param._cpu_grad  # type: ignore[attr-defined]
                # If user specified to keep the gradient in low precision, then
                # the gradient may still be of the low precision dtype if the
                # user did not set the gradient to `None` after the previous
                # backward, in which case FSDP should cast back to the full
                # precision dtype so that FSDP can accumulate in that dtype in
                # the post-backward hook and assign to `.grad` in that dtype in
                # the post-backward callback.
                local_shard_dtype = flat_param._local_shard.dtype  # type: ignore[attr-defined]
                if (
                    self._keep_low_precision_grads
                    and sharded_grad.dtype != local_shard_dtype
                ):
                    sharded_grad.data = sharded_grad.to(local_shard_dtype)
            else:
                padded_unsharded_size = flat_param._padded_unsharded_size  # type: ignore[attr-defined]
                _p_assert(
                    flat_param.grad.size() == padded_unsharded_size,
                    "Expects `.grad` to be the unsharded gradient in "
                    f"`no_sync()` with size {padded_unsharded_size} "
                    f"but got size {flat_param.grad.size()}",
                )
            flat_param.grad = None

    def prepare_gradient_for_optim(self):
        """
        Prepares the gradient for optimizer computation by moving the sharded
        gradient to the ``.grad`` attribute.
        """

        def cast_grad_to_param_dtype_if_needed(flat_param):
            if self._keep_low_precision_grads:
                assert flat_param.grad is not None  # mypy
                if flat_param.grad.dtype != self._fwd_bwd_param_dtype:
                    flat_param.grad.data = flat_param.grad.to(self._fwd_bwd_param_dtype)
                    if self._use_orig_params:
                        self._use_sharded_grad_views()

        flat_param = self.flat_param
        # TODO (awgu): We should replace these conditional checks to encode
        # the logical intention more directly.
        if hasattr(flat_param, "_cpu_grad"):
            # NOTE: This branch includes `NO_SHARD`.
            self._check_sharded(flat_param)
            self._check_on_cpu(flat_param)
            flat_param.grad = flat_param._cpu_grad  # type: ignore[attr-defined]
            cast_grad_to_param_dtype_if_needed(flat_param)
        elif hasattr(flat_param, "_saved_grad_shard"):
            self._check_sharded(flat_param)
            self._check_on_compute_device(flat_param)
            self._check_on_compute_device(flat_param._saved_grad_shard)  # type: ignore[attr-defined]
            # If no sharded gradient was computed this iteration, then there is
            # no need to forward `_saved_grad_shard` to `grad`
            if flat_param._post_backward_called:  # type: ignore[attr-defined]
                flat_param.grad = flat_param._saved_grad_shard  # type: ignore[attr-defined]
                cast_grad_to_param_dtype_if_needed(flat_param)
        else:
            _p_assert(
                not self.uses_sharded_strategy
                or not flat_param._post_backward_called,  # type: ignore[attr-defined]
                "All sharded parameters that received a gradient in the "
                "post-backward should use `_saved_grad_shard`",
            )
        # Delete `_saved_grad_shard` since its existence indicates a previous
        # gradient to accumulate with in the post-backward hook
        if hasattr(flat_param, "_saved_grad_shard"):
            delattr(flat_param, "_saved_grad_shard")

    @contextlib.contextmanager
    def to_cpu(self):
        """
        Moves the unpadded unsharded flattened parameter to CPU while in the
        context and moves it back to the previous device upon exit. For now,
        this assumes the ``FlatParameter`` is the unpadded unsharded flattened
        parameter since (1) there is no reason to include the padding in the
        copy and (2) there is no use case for the sharded flattened parameter.

        Precondition: ``self.flat_param`` 's data is the unpadded unsharded
        flattened parameter on the compute device, and the handle uses a
        sharded strategy.
        Postcondition: Same as the precondition.
        """
        self._check_sharded_strategy()
        _p_assert(
            self.flat_param.size() == self.flat_param._unpadded_unsharded_size,
            f"Expects size {self.flat_param._unpadded_unsharded_size} but got {self.flat_param.size()}",
        )
        self._check_on_compute_device(self.flat_param)
        # Check that the unpadded unsharded flattened parameter is a view into
        # the padded unsharded flattened parameter as expected
        # NOTE: This check is not strictly needed for correctness but is a
        # useful sanity check since the tensor should only be used internally.
        unpadded_storage_ptr = self.flat_param._typed_storage()._data_ptr()
        padded_storage_ptr = (
            self._get_padded_unsharded_flat_param()._typed_storage()._data_ptr()
        )
        _p_assert(
            unpadded_storage_ptr == padded_storage_ptr,
            "Expects the unpadded parameter to be a view into the padded parameter",
        )
        self.flat_param_to(torch.device("cpu"))
        self._free_unsharded_flat_param()
        try:
            yield
        finally:
            _p_assert(
                self.flat_param.size() == self.flat_param._unpadded_unsharded_size,
                f"Expects size {self.flat_param._unpadded_unsharded_size} but got {self.flat_param.size()}",
            )
            padded_unsharded_flat_param = self._alloc_padded_unsharded_flat_param()
            # Copy from CPU to the compute device
            padded_unsharded_flat_param[: self.flat_param.numel()].copy_(
                self.flat_param
            )
            self._use_unsharded_flat_param(padded_unsharded_flat_param)

    def reshard(self, free_unsharded_flat_param: bool):
        """
        Runs the reshard logic. This includes freeing the unsharded flattened
        parameter if ``free_unsharded_flat_param`` and switching to using the
        sharded flattened parameter.
        """
        # Switch to the sharded `FlatParameter` before freeing to prevent
        # "use-after-free"-type bugs with external profiling tools, where for
        # `use_orig_params=True`, the `param` does not point to valid memory
        # when setting `param.data = ...` in `_use_sharded_views()`.
        self._use_sharded_flat_param()
        if free_unsharded_flat_param:
            self._free_unsharded_flat_param()

    def post_reshard(self):
        """
        Runs the post-reshard logic. This includes freeing any memory that
        can now be freed given that the ``FlatParameter`` points to the full
        precision sharded flattened parameter.

        Precondition: ``self.flat_param`` 's data points to the full precision
        sharded flattened parameter.
        """
        # For `NO_SHARD`, `_mp_shard` is not freed in the post-unshard since
        # it is also the low precision *unsharded* flattened parameter. Hence,
        # we delay the free until the reshard.
        if (
            self._uses_param_mixed_precision
            and not self.uses_sharded_strategy
            and not self._force_full_precision  # did not use the low precision shard
        ):
            self._free_low_precision_sharded_param()

    def _free_unsharded_flat_param(self):
        """
        Frees the padded unsharded flattened parameter. The tensor to free
        depends on the calling context since the unshard may have forced full
        precision, in which case a different tensor is used.
        """
        self._check_sharded_strategy()
        unsharded_flat_param = self._get_padded_unsharded_flat_param()
        self._check_storage_allocated(unsharded_flat_param)
        self._check_on_compute_device(unsharded_flat_param)
        # Do not free the memory until all ops in the current stream finish
        _no_dispatch_record_stream(unsharded_flat_param, torch.cuda.current_stream())
        _free_storage(unsharded_flat_param)

    def _use_sharded_flat_param(self) -> None:
        """Switches to using the sharded flattened parameter."""
        flat_param = self.flat_param
        if self._offload_params:
            device = flat_param._local_shard.device  # type: ignore[attr-defined]
            _p_assert(
                device == torch.device("cpu"),
                f"Expects the local shard to be on CPU but got {device}",
            )
        flat_param.data = flat_param._local_shard  # type: ignore[attr-defined]
        if self._use_orig_params:
            self._use_sharded_views()
            # For the post-forward reshard, we may try to use sharded gradient
            # views (or unsharded gradient views if a gradient was accumulated
            # in `no_sync()`), but for the post-backward reshard, we delay the
            # call to after the reduce-scatter.
            if self._training_state == HandleTrainingState.FORWARD:
                # TODO: Change `_unpadded_unsharded_size` if we change the
                # gradient to be computed directly with padding.
                accumulated_grad_in_no_sync = (
                    flat_param.grad is not None
                    and self.uses_sharded_strategy
                    and flat_param.grad.shape == flat_param._unpadded_unsharded_size
                )
                if accumulated_grad_in_no_sync:
                    self._use_unsharded_grad_views()
                else:
                    self._use_sharded_grad_views()

    #########
    # VIEWS #
    #########
    @staticmethod
    def _get_unflat_views(
        flat_param: FlatParameter,
        tensor: Optional[torch.Tensor] = None,
    ) -> Iterator[Tensor]:
        """
        Returns unflattened ``Tensor`` views into ``tensor`` if it is not
        ``None`` or ``flat_param`` otherwise, where the unflattening is based
        on ``flat_param`` 's metadata.

        In other words, to get views into the unsharded flattened parameter,
        pass ``tensor`` as ``None``, but to get views into tensor optimizer
        state, pass ``tensor`` as the optimizer state tensor.
        """
        if tensor is None:
            tensor = flat_param
        _p_assert(
            tensor.numel() == flat_param._unpadded_unsharded_size.numel(),
            f"Expects {flat_param._unpadded_unsharded_size.numel()} numel but got "
            f"{tensor.numel()} numel",
        )
        views = (
            _ext_post_unflatten_transform(subtensor.view(shape), param_extension)
            for (subtensor, shape, param_extension) in zip(
                torch.split(tensor, flat_param._numels, dim=0),  # type: ignore[arg-type]
                flat_param._shapes,
                flat_param._param_extensions,
            )
        )
        return views

    def _use_unsharded_views(self, as_params: bool) -> None:
        """
        Unflattens the unsharded flattened parameter by setting the original
        module parameter variables to be views into it.

        Args:
            as_params (bool): If ``True``, then registers the original
                parameters as ``nn.Parameter`` s; if ``False``, then registers
                the original parameters only as ``Tensor`` s. ``False`` should
                be used during forward/backward computation and when hiding the
                original parameters from :meth:`nn.Module.named_parameters`.
        """
        self._check_unsharded(self.flat_param)
        views = self._get_unflat_views(self.flat_param)
        for i, (view, (param_name, module, _)) in enumerate(
            zip(views, self.flat_param._param_infos)
        ):
            if self._use_orig_params and as_params:
                if type(view) is DTensor:
                    # A `DTensor` `view` is not compatible with assigning
                    # `param.data = view`, so we cannot preserve the parameter
                    # variable.
                    self._setattr_param(module, param_name, nn.Parameter(view))
                    continue
                param = self.flat_param._params[i]  # type: ignore[index]
                self._setattr_param(module, param_name, param)
                param.data = view
            elif as_params:
                self._setattr_param(module, param_name, nn.Parameter(view))
            else:  # `as_params=False`
                param_var: Tensor = view
                if self._use_orig_params:
                    if self._training_state == HandleTrainingState.FORWARD:
                        assert self.flat_param._tensors is not None
                        # Save the `Tensor` for the pre-backward
                        self.flat_param._tensors[i] = view  # save for pre-backward
                    elif self._training_state == HandleTrainingState.BACKWARD_PRE:
                        # Use the saved `Tensor` variable from the forward to
                        # preserve the autograd graph so that the post-backward
                        # hook fires (e.g. for reentrant AC)
                        assert self.flat_param._tensors is not None  # mypy
                        tensor = self.flat_param._tensors[i]
                        _p_assert(
                            tensor is not None,
                            "Expects `Tensor` to have been saved in forward",
                        )
                        tensor.data = view  # type: ignore[union-attr]
                        assert tensor is not None  # mypy
                        param_var = tensor
                self._setattr_tensor(module, param_name, param_var)
                if (
                    self._use_orig_params
                    and self._training_state == HandleTrainingState.FORWARD
                ):
                    module._parameters[param_name] = param_var  # type: ignore[assignment]
        for i, (
            param_name,
            module,
            _,
            prim_param_name,
            prim_module,
            prim_module_name,
        ) in enumerate(self.flat_param._shared_param_infos):
            if hasattr(module, param_name):
                delattr(module, param_name)
            _p_assert(
                hasattr(prim_module, prim_param_name),
                f"Module {prim_module_name} is missing parameter {prim_param_name}",
            )
            prim_param: Union[Tensor, nn.Parameter] = getattr(
                prim_module, prim_param_name
            )
            _p_assert(
                not as_params or isinstance(prim_param, nn.Parameter),
                f"as_params={as_params} type(prim_param)={type(prim_param)}",
            )
            if self._use_orig_params and as_params:
                shared_param = self.flat_param._shared_params[i]  # type: ignore[index]
                self._setattr_param(module, param_name, shared_param)
                shared_param.data = prim_param
            elif as_params:
                assert isinstance(prim_param, nn.Parameter)
                self._setattr_param(module, param_name, prim_param)
            else:
                self._setattr_tensor(module, param_name, prim_param)
                if (
                    self._use_orig_params
                    and self._training_state == HandleTrainingState.FORWARD
                ):
                    module._parameters[param_name] = prim_param  # type: ignore[assignment]

    def _use_unsharded_grad_views(self) -> None:
        """
        Unflattens the unsharded flattened parameter's gradient by setting the
        original module parameter variables' gradients to be views into it.
        """
        # Expects the gradient to be in `flat_param.grad`
        if self.flat_param.grad is None:
            assert self.flat_param._params is not None  # mypy
            assert self.flat_param._shared_params is not None  # mypy
            for param in chain(
                self.flat_param._params,  # type: ignore[attr-defined]
                self.flat_param._shared_params,  # type: ignore[attr-defined]
            ):
                param.grad = None
            return
        self._check_unsharded(self.flat_param.grad)
        views = self._get_unflat_views(self.flat_param, self.flat_param.grad)
        for i, (view, (param_name, module, _)) in enumerate(
            zip(views, self.flat_param._param_infos)
        ):
            _p_assert(
                hasattr(module, param_name),
                f"{self.flat_param._fqns[i]} is missing",
            )
            param = getattr(module, param_name)
            if param.shape != view.shape or param.dtype != view.dtype:
                # NOTE: This is a hack using `.data` to side step the
                # check that parameter/gradient sizes and dtypes match. Here,
                # `param` can have the sharded size, and `grad` can have the
                # unsharded size. Orthgonally, `param` can have the full
                # precision dtype from `reshard()`, and `grad` can have the
                # parameter low precision dtype. Both of these mismatches
                # happen when running in `no_sync()`.
                if param.grad is None:
                    param.grad = torch.empty_like(param)
                param.grad.data = view
            else:
                param.grad = view
        for i, (
            param_name,
            module,
            module_name,
            prim_param_name,
            prim_module,
            _,
        ) in enumerate(self.flat_param._shared_param_infos):
            _p_assert(
                hasattr(module, param_name),
                f"{module_name + '.' + param_name if module_name else param_name} is missing",
            )  # did not save FQN info in `_shared_param_infos`
            param = getattr(module, param_name)
            prim_param = getattr(prim_module, prim_param_name)
            if (
                param.shape != prim_param.grad.shape
                or param.dtype != prim_param.grad.dtype
            ):
                # NOTE: This is the same hack to use `.data` to side step the
                # size check.
                if param.grad is None:
                    param.grad = torch.empty_like(param)
                param.grad.data = prim_param.grad
            else:
                param.grad = prim_param.grad

    @contextlib.contextmanager
    def unflatten_as_params(self) -> Generator:
        """
        Assumes the flattened parameter is unsharded. When in the context,
        unflattens the original parameters as ``nn.Parameter`` views into the
        flattened parameter, and after the context, restores the original
        parameters as ``Tensor`` views into the flattened parameter.
        """
        self._use_unsharded_views(as_params=True)
        try:
            yield
        finally:
            self._use_unsharded_views(as_params=False)

    @torch.no_grad()
    def _use_sharded_views(self) -> None:
        """
        Sets the original module parameter variables' data to be flattened
        views into the sharded flattened parameter.

        The views are kept as flattened to simplify the case where a parameter
        is sharded across ranks. Parameters whose data is not present in the
        sharded flattened parameter have their data set to a size-0 empty
        tensor. We do not delete them to ensure to preserve expected behaviors
        like model printability. Parameters whose data is present must preserve
        their variables to be passable to an optimizer.
        """
        if not self.uses_sharded_strategy:
            # For `NO_SHARD`, use the *unflattened* unsharded views since we
            # have the unsharded parameter
            self._use_unsharded_views(as_params=True)
            return
        self._check_sharded(self.flat_param)
        start, end = self.flat_param._shard_indices  # type: ignore[attr-defined]
        offset = 0
        assert self.flat_param._params is not None
        for i, (param, (param_name, module, _)) in enumerate(
            zip(self.flat_param._params, self.flat_param._param_infos)
        ):
            self._setattr_param(module, param_name, param)
            in_sharded_flat_param = (
                i >= start
                and i <= end
                and self.flat_param._shard_param_offsets  # type: ignore[attr-defined]
            )
            if in_sharded_flat_param:
                param_start, param_end = self.flat_param._shard_param_offsets[i - start]  # type: ignore[attr-defined]
                numel_in_shard = param_end - param_start + 1
                param.data = self.flat_param[offset : offset + numel_in_shard]
                offset += numel_in_shard
            else:
                # Allow the original data to be freed via garbage collection
                param.data = torch.empty(
                    0,
                    dtype=self.flat_param.dtype,  # in case `flat_param` changed dtype
                    device=self.flat_param.device,
                    requires_grad=False,
                )
        assert self.flat_param._shared_params is not None
        for i, (
            param,
            (param_name, module, _, prim_param_name, prim_module, _),
        ) in enumerate(
            zip(self.flat_param._shared_params, self.flat_param._shared_param_infos)
        ):
            self._setattr_param(module, param_name, param)
            prim_param = getattr(prim_module, prim_param_name)
            param.data = prim_param  # could be both empty and non-empty
        if self._training_state == HandleTrainingState.BACKWARD_POST:
            assert self.flat_param._tensors is not None  # mypy
            # Clear the saved `Tensor`s since they are unneeded now
            for i in range(len(self.flat_param._tensors)):
                self.flat_param._tensors[i] = None  # type: ignore[index]

    @torch.no_grad()
    def _use_sharded_grad_views(self) -> None:
        """
        Sets the original module parameter variables' gradients to be flattened
        views into the sharded flattened parameter's gradient. This is a no-op
        if there is no gradient.

        Parameters whose data is not present in the sharded flattened parameter
        and parameters with ``requires_grad=False`` have their gradients set to
        ``None``. Since the gradient variables do not need to be preserved,
        this method does not manipulate existing ``Tensor`` data directly and
        creates new ``Tensor`` variables instead.
        """
        flat_param = self.flat_param
        self._check_sharded(flat_param)
        grad = self.sharded_grad
        if grad is None:
            assert flat_param._params is not None  # mypy
            assert flat_param._shared_params is not None  # mypy
            for param in chain(flat_param._params, flat_param._shared_params):  # type: ignore[attr-defined]
                param.grad = None
            return
        self._check_sharded(grad)
        start, end = flat_param._shard_indices  # type: ignore[attr-defined]
        offset = 0
        assert flat_param._params is not None
        for i, param in enumerate(flat_param._params):
            in_sharded_flat_param = (
                i >= start
                and i <= end
                and flat_param._shard_param_offsets  # type: ignore[attr-defined]
            )
            if in_sharded_flat_param:
                param_start, param_end = flat_param._shard_param_offsets[i - start]  # type: ignore[attr-defined]
                numel_in_shard = param_end - param_start + 1
                assert flat_param._is_grad_none is not None  # mypy
                if param.requires_grad and not flat_param._is_grad_none[i]:
                    if self._keep_low_precision_grads or param.dtype != grad.dtype:
                        # NOTE: This is a hack using `.data` to side step the
                        # check that parameter/gradient dtypes match. Here,
                        # `param` has full precision; `grad` has low precision.
                        if param.grad is None:
                            # `.grad` must have the same shape as `param`
                            param.grad = torch.empty_like(param)
                        param.grad.data = grad[
                            offset : offset + numel_in_shard
                        ].reshape(param.shape)
                    else:
                        param.grad = grad[offset : offset + numel_in_shard].reshape(
                            param.shape
                        )
                else:
                    param.grad = None
                offset += numel_in_shard
            else:
                param.grad = None
        assert flat_param._shared_params is not None
        for i, (param, (_, _, _, prim_param_name, prim_module, _)) in enumerate(
            zip(flat_param._shared_params, flat_param._shared_param_infos)
        ):
            in_sharded_flat_param = hasattr(prim_module, prim_param_name)
            if in_sharded_flat_param and param.requires_grad:
                prim_param = getattr(prim_module, prim_param_name)
                param.grad = prim_param.grad  # share the same reference
            else:
                param.grad = None

    @torch.no_grad()
    def _writeback_orig_params(self) -> bool:
        """
        Iterates over the original parameters and writes back any parameters
        that changed storages (due to a non-inplace operator) to the handle's
        ``FlatParameter``. This method preserves the ``FlatParameter` 's
        device even if an original parameter's device changes.

        Raises:
            RuntimeError: If an original parameter or gradient changes storages
            but no longer has the expected flattened shape.
        Returns: ``True`` if some writeback happened, and ``False`` otherwise.
        """
        if self.uses_sharded_strategy and not self.is_sharded(self.flat_param):
            # For `NO_SHARD`, we may still need to writeback
            return False
        flat_param = self.flat_param
        start, end = flat_param._shard_indices  # type: ignore[attr-defined]
        offset = 0
        assert flat_param._params is not None
        wroteback = False
        for i, (param, (param_name, module, _)) in enumerate(
            zip(flat_param._params, flat_param._param_infos)
        ):
            if not hasattr(module, param_name):
                # Do not writeback if original parameters are deregistered
                # (e.g. during model checkpointing)
                continue
            in_sharded_flat_param = (
                i >= start
                and i <= end
                and self.flat_param._shard_param_offsets  # type: ignore[attr-defined]
            )
            if not in_sharded_flat_param:
                continue
            param_start, param_end = flat_param._shard_param_offsets[i - start]  # type: ignore[attr-defined]
            numel_in_shard = param_end - param_start + 1

            # Check for parameter writeback
            param_changed = getattr(module, param_name) is not param
            needs_param_writeback = (
                param_changed  # changed parameter variable itself
                or not _same_storage(param, flat_param)  # changed `.data`
            )
            if param_changed:
                # NOTE: The gradient is not preserved after a parameter change.
                param = getattr(module, param_name)
                flat_param._params[i] = param
            if needs_param_writeback:
                expected_shape = torch.Size([numel_in_shard])
                self._writeback_tensor(
                    param, flat_param, i, expected_shape, offset, True
                )
                wroteback = True

            # Check for gradient writeback
            # NOTE: Since this method is called in the pre-unshard, which is
            # only called during computation in the pre-forward or
            # pre-backward, the sharded gradient should be guaranteed to be in
            # `.grad`, not in `._saved_grad_shard`.
            if param.grad is None and flat_param.grad is not None:
                expected_shape = torch.Size([numel_in_shard])
                self._writeback_tensor(
                    None, flat_param.grad, i, expected_shape, offset, False
                )
            elif param.grad is not None:
                # For `NO_SHARD` + CPU offloading, `_cpu_grad` is always in
                # memory and owns the gradient storage, so it will never
                # require gradient writeback.
                flat_param_grad = (
                    flat_param.grad
                    if self.uses_sharded_strategy or not self._offload_params
                    else flat_param._cpu_grad  # type: ignore[attr-defined]
                )
                needs_grad_writeback = flat_param_grad is None or not _same_storage(
                    param.grad, flat_param_grad
                )
                if needs_grad_writeback:
                    if flat_param_grad is None:
                        flat_param_grad = torch.zeros_like(flat_param)
                    expected_shape = torch.Size([numel_in_shard])
                    self._writeback_tensor(
                        param.grad, flat_param_grad, i, expected_shape, offset, False
                    )
                    flat_param.grad = flat_param_grad
            offset += numel_in_shard
        # TODO (awgu): Handle shared parameters. We need to re-generate the
        # shared parameter data structures in case sharedness changed.
        for i, (
            param_name,
            module,
            _,
            prim_param_name,
            prim_module,
            _,
        ) in enumerate(flat_param._shared_param_infos):
            if getattr(module, param_name) is not getattr(prim_module, prim_param_name):
                raise NotImplementedError(
                    "Changing shared parameters is not supported yet"
                )
        return wroteback

    def _writeback_tensor(
        self,
        src_tensor: Optional[Tensor],
        dst_tensor: Tensor,
        tensor_index: int,
        expected_shape: torch.Size,
        offset: int,
        is_param: bool,  # else gradient
    ) -> None:
        """
        Writes back ``src_tensor`` to ``dst_tensor`` at offset ``offset``,
        where ``src_tensor`` should have shape ``expected_shape``. ``is_param``
        indicates if the tensor is the parameter (if ``True``) or gradient (if
        ``False``). If ``src_tensor`` is ``None``, then the effect is zeroing
        instead of copying. ``tensor_index`` gives the index of ``src_tensor``
        in the metadata structures.

        Raises:
            RuntimeError: If the ``src_tensor`` does not have the expected
            shape.
        """
        _p_assert(
            len(expected_shape) == 1,
            f"Expects a 1D expected shape but got {expected_shape}",
        )
        if self._debug_level == dist.DebugLevel.DETAIL:
            rank = self.rank if hasattr(self, "rank") else dist.get_rank()
            src_shape = src_tensor.shape if src_tensor is not None else None
            src_device = src_tensor.device if src_tensor is not None else None
            warnings.warn(
                f"[Rank {rank}] {'Parameter' if is_param else 'Gradient'} needs "
                f"writeback in {self._training_state}\n"
                f"expected shape={expected_shape} shape={src_shape} "
                f"expected device={dst_tensor.device} device={src_device}"
            )
        if src_tensor is not None and src_tensor.shape != expected_shape:
            # NOTE: Gradient shape mismatch is not possible in practice since
            # the gradient shape is enforced to match that of the parameter and
            # we already check for parameter shape mismatch.
            raise RuntimeError(
                f"Cannot writeback when the {'parameter' if is_param else 'gradient'} "
                f"shape changes\nExpects {expected_shape} but got {src_tensor.shape}"
            )
        if src_tensor is not None:
            dst_tensor[offset : offset + expected_shape.numel()].copy_(src_tensor)
        else:
            dst_tensor[offset : offset + expected_shape.numel()].zero_()
            assert self.flat_param._is_grad_none is not None
            self.flat_param._is_grad_none[tensor_index] = True

    def _clear_grads_if_needed(self):
        """
        When ``use_orig_params=True``, sets the underlying ``flat_param.grad``
        to ``None`` if *all* of the original parameters' ``.grad`` are
        ``None``. This is targeting ``optim.zero_grad(set_to_none=True)``, in
        which case we want to free the gradients as soon after the
        ``zero_grad()`` call as possible.
        """
        if not self._use_orig_params:
            return
        flat_param = self.flat_param
        assert flat_param._params is not None
        if all(param.grad is None for param in flat_param._params):
            flat_param.grad = None

    def _deregister_orig_params(self):
        for param_name, module, _ in self.flat_param._param_infos:
            if hasattr(module, param_name):
                delattr(module, param_name)
        for param_name, module, _, _, _, _ in self.flat_param._shared_param_infos:
            if hasattr(module, param_name):
                delattr(module, param_name)

    ###########
    # HELPERS #
    ###########
    def flat_param_to(self, *args, **kwargs):
        """Wraps an in-place call to ``.to()`` for ``self.flat_param``."""
        self.flat_param.data = self.flat_param.to(*args, **kwargs)
        if self._use_orig_params:
            # Refresh the views because their storage may have changed
            if self.is_sharded(self.flat_param):
                self._use_sharded_views()
            else:
                self._use_unsharded_views(as_params=True)

    def _get_modules(self) -> Set[nn.Module]:
        """Returns a :class:`set` of the modules whose parameters are included
        in this handle's flattened parameter."""
        return {pi.module for pi in self.flat_param._param_infos}.union(
            {spi.module for spi in self.flat_param._shared_param_infos}
        )

    def is_sharded(self, tensor: Tensor) -> bool:
        """
        Returns if ``tensor`` is *currently* sharded. For ``NO_SHARD``, we
        choose to have this always return ``False`` for clarity.
        """
        if (
            not hasattr(self.flat_param, "_sharded_size")
            or not self.uses_sharded_strategy
        ):
            # `_sharded_size` is defined iff `handle.shard()` has been called
            return False
        sharded_size = self.flat_param._sharded_size  # type: ignore[attr-defined]
        return tensor.size() == sharded_size

    def parameter_module_names(self) -> Iterator[Tuple[str, str]]:
        shared_param_infos = [
            ParamInfo(param_name, module, module_name)
            for (
                param_name,
                module,
                module_name,
                _,
                _,
                _,
            ) in self.flat_param._shared_param_infos
        ]
        for param_name, _, module_name in chain(
            self.flat_param._param_infos, shared_param_infos
        ):
            yield (param_name, module_name)

    def shared_parameter_module_names(self) -> Iterator[Tuple[str, str]]:
        for param_name, _, module_name in [
            ParamInfo(param_name, module, module_name)
            for (
                param_name,
                module,
                module_name,
                _,
                _,
                _,
            ) in self.flat_param._shared_param_infos
        ]:
            yield (param_name, module_name)

    @property
    def _fqns_in_shard(self) -> List[str]:
        """Returns the FQNs of the parameters present in this rank's shard."""
        fqns_in_shard: List[str] = []
        start, end = self.flat_param._shard_indices  # type: ignore[attr-defined]
        for i in range(len(self.flat_param._fqns)):
            if i >= start and i <= end and self.flat_param._shard_param_offsets:  # type: ignore[attr-defined]
                fqns_in_shard.append(self.flat_param._fqns[i])
        return fqns_in_shard

    @property
    def sharded_grad(self) -> Optional[Tensor]:
        """Returns the handle's sharded gradient."""
        flat_param = self.flat_param
        # Priority for non-`None`: `_cpu_grad` > `_saved_grad_shard` > `grad`
        # - CPU offloading: `_cpu_grad`
        # - No CPU offloading + sharded strategies: `_saved_grad_shard`
        # - No CPU offloading + `NO_SHARD`: `grad`
        if hasattr(flat_param, "_cpu_grad"):
            grad = flat_param._cpu_grad  # type: ignore[attr-defined]
        elif hasattr(flat_param, "_saved_grad_shard"):
            # In the post-backward hook, the sharded gradient is still in
            # `_saved_grad_shard`.
            grad = flat_param._saved_grad_shard  # type: ignore[attr-defined]
        else:
            # If in IDLE or in FORWARD states, then there may be an
            # (accumulated) gradient. If accessed in IDLE, then this should
            # be due to re-registering the original parameters (e.g. in state
            # dict load).
            _p_assert(
                flat_param.grad is None
                or not self.uses_sharded_strategy
                or self._training_state
                in (HandleTrainingState.FORWARD, HandleTrainingState.IDLE),
                "Sharded strategies should use `_cpu_grad` or `_saved_grad_shard` "
                "unless in IDLE or FORWARD",
            )
            grad = flat_param.grad
        return grad

    def _reset_is_grad_none(self) -> None:
        """
        Resets the ``_is_grad_none`` mask as needed. This method should only be
        called in the post-backward after gradient computation, in which case
        if a parameter requires gradient, then it will surely receive a
        gradient and we may reset its mask entry to ``False``.
        """
        if not self._use_orig_params:
            return
        _p_assert(
            self._training_state == HandleTrainingState.BACKWARD_POST,
            "Expects to only be called in the post-backward after gradient computation",
        )
        flat_param = self.flat_param
        assert flat_param._params is not None  # mypy
        for i, param in enumerate(flat_param._params):
            # As long as the parameter requires gradient, it should receive a
            # meaningful gradient (even if the gradient happens to be zeros)
            if param.requires_grad:
                assert flat_param._is_grad_none is not None  # mypy
                flat_param._is_grad_none[i] = False

    #######################
    # CHECKS & INVARIANTS #
    #######################
    def _check_sharded_strategy(self):
        _p_assert(self.uses_sharded_strategy, "Expects sharded strategy")

    def _check_on_compute_device(self, tensor: Tensor):
        _p_assert(
            tensor.device == self.device,
            f"Expects tensor to be on the compute device {self.device}",
        )

    def _check_on_cpu(self, tensor: Tensor):
        _p_assert(
            tensor.device == torch.device("cpu"),
            f"Expects tensor to be on CPU but got {tensor.device}",
        )

    @staticmethod
    def _check_storage_freed(tensor: Tensor):
        storage_size: int = tensor._typed_storage()._size()
        _p_assert(
            storage_size == 0,
            f"Expects storage to be freed but got storage with size {storage_size}",
        )

    @staticmethod
    def _check_storage_allocated(tensor: Tensor):
        storage_size: int = tensor._typed_storage()._size()
        _p_assert(storage_size > 0, "Expects storage to be allocated")

    def _check_low_precision_shard(self):
        _p_assert(
            self._uses_param_mixed_precision,
            "Not using low precision for parameters",
        )
        _p_assert(
            getattr(self.flat_param, "_mp_shard", None) is not None,
            "Expects `_mp_shard` to exist",
        )
        device = self.flat_param._mp_shard.device  # type: ignore[attr-defined]
        _p_assert(
            device == self.device,
            f"Expects the low precision shard to be on {self.device} but got {device}",
        )

    def _check_unsharded(self, tensor: Tensor):
        msg_prefix = "Expects tensor to be unsharded "
        _p_assert(tensor is not None, msg_prefix + "but got `None`")
        unsharded_size = self.flat_param._unpadded_unsharded_size
        _p_assert(
            tensor.size() == unsharded_size,
            msg_prefix + f"with size {unsharded_size} but got {tensor.size()}",
        )

    def _check_sharded(self, tensor: Tensor):
        msg_prefix = "Expects tensor to be sharded "
        _p_assert(tensor is not None, msg_prefix + "but got `None`")
        sharded_size = self.flat_param._sharded_size  # type: ignore[attr-defined]
        _p_assert(
            tensor.size() == sharded_size,
            msg_prefix + f"with size {sharded_size} but got {tensor.size()}",
        )

    ##############
    # PROPERTIES #
    ##############
    @property
    def uses_sharded_strategy(self) -> bool:
        return self._sharding_strategy != HandleShardingStrategy.NO_SHARD

    @property
    def _uses_param_mixed_precision(self) -> bool:
        return self._fwd_bwd_param_dtype != self._orig_param_dtype

    @property
    def _uses_reduce_mixed_precision(self) -> bool:
        return self._reduce_dtype != self._orig_param_dtype

    @property
    def _force_full_precision(self) -> bool:
        return (
            self._training_state == HandleTrainingState.SUMMON_FULL_PARAMS
            and self._uses_param_mixed_precision
        )


# NOTE: These are hacks to bypass `nn.Module.__setattr__` checks.
def _unsafe_setattr_param(
    module: nn.Module, param_name: str, param: nn.Parameter
) -> None:
    module._parameters[param_name] = param
    # This bypasses any overrides in case `module` is an instance of an
    # `nn.Module` subclass
    super(nn.Module, module).__setattr__(param_name, param)


def _unsafe_setattr_tensor(module: nn.Module, param_name: str, tensor: Tensor) -> None:
    module._parameters.pop(param_name, None)
    # This bypasses any overrides in case `module` is an instance of an
    # `nn.Module` subclass
    super(nn.Module, module).__setattr__(param_name, tensor)


def _safe_setattr_tensor_or_param(
    module: nn.Module, param_name: str, tensor_or_param: Union[Tensor, nn.Parameter]
):
    # Call `delattr()` and `setattr()` to go through `nn.Module` checks
    if hasattr(module, param_name):
        delattr(module, param_name)
    setattr(module, param_name, tensor_or_param)


# A handles key represents the group of `FlatParamHandle`s involved in a given
# module's forward. These will be all-gathered together in the pre-forward and
# pre-backward.
_HandlesKey = Tuple[FlatParamHandle, ...]