[ONNX] Support float4 (#151069)

- Support exporting float4 models (note: currently we use IR version 10 universally in the exporter, which does not include float 4 support. Eventually when onnx runtime and the ecosystem moves to support the new IR version 11 we should bump our version to 11 in the exporter as well)
- The shape of the type is set according to https://github.com/pytorch/pytorch/pull/148791#discussion_r2038704986 (added last dim with size 2)
- Use ml_dtypes types when converting to numpy for consistency with ONNX IR

Fix https://github.com/pytorch/pytorch/issues/150202

Pull Request resolved: https://github.com/pytorch/pytorch/pull/151069
Approved by: https://github.com/titaiwangms

test/onnx/exporter/test_core.py

@@ -35,9 +35,13 @@ class TorchTensorTest(common_utils.TestCase):
             (torch.uint32, np.uint32),
             (torch.uint64, np.uint64),
             (torch.uint8, np.uint8),
+            (torch.float4_e2m1fn_x2, ml_dtypes.float4_e2m1fn),
         ],
     )
     def test_numpy_returns_correct_dtype(self, dtype: torch.dtype, np_dtype):
+        if dtype == torch.float4_e2m1fn_x2:
+            tensor = _core.TorchTensor(torch.tensor([1], dtype=torch.uint8).view(dtype))
+        else:
             tensor = _core.TorchTensor(torch.tensor([1], dtype=dtype))
         self.assertEqual(tensor.numpy().dtype, np_dtype)
         self.assertEqual(tensor.__array__().dtype, np_dtype)
@@ -71,6 +75,12 @@ class TorchTensorTest(common_utils.TestCase):
         tensor = _core.TorchTensor(torch.tensor([1], dtype=dtype))
         self.assertEqual(tensor.tobytes(), tensor.numpy().tobytes())
 
+    def test_tobytes_float4(self):
+        tensor = _core.TorchTensor(
+            torch.tensor([1], dtype=torch.uint8).view(torch.float4_e2m1fn_x2)
+        )
+        self.assertEqual(tensor.tobytes(), b"\x01")
+
 
 if __name__ == "__main__":
     common_utils.run_tests()

test/onnx/exporter/test_small_models_e2e.py

@@ -216,7 +216,19 @@ class DynamoExporterTest(common_utils.TestCase):
                 input = input.to(float8_type)
                 return input
 
-        _ = self.export(Float8Module(), (torch.randn(1, 2),))
+        onnx_program = self.export(Float8Module(), (torch.randn(1, 2),))
+        self.assertEqual(onnx_program.model.graph.outputs[0].dtype, onnx_type)
+
+    def test_float4_support(self):
+        class Float4Module(torch.nn.Module):
+            def forward(self):
+                return torch.empty([1], dtype=torch.float4_e2m1fn_x2)
+
+        onnx_program = self.export(Float4Module())
+        output = onnx_program.model.graph.outputs[0]
+        self.assertEqual(output.dtype, ir.DataType.FLOAT4E2M1)
+        # The shape is [*shape, 2] because ONNX stores the shape of the unpacked tensor
+        self.assertEqual(output.shape.dims, [1, 2])
 
     def test_bfloat16_support(self):
         class BfloatModel(torch.nn.Module):

torch/onnx/_internal/exporter/_core.py

@@ -38,6 +38,7 @@ from torch.onnx._internal.exporter import (
     _registration,
     _reporting,
     _tensors,
+    _type_casting,
     _verification,
 )
 
@@ -61,6 +62,7 @@ _TORCH_DTYPE_TO_ONNX: dict[torch.dtype, ir.DataType] = {
     torch.float8_e4m3fnuz: ir.DataType.FLOAT8E4M3FNUZ,
     torch.float8_e5m2: ir.DataType.FLOAT8E5M2,
     torch.float8_e5m2fnuz: ir.DataType.FLOAT8E5M2FNUZ,
+    torch.float4_e2m1fn_x2: ir.DataType.FLOAT4E2M1,
     torch.int16: ir.DataType.INT16,
     torch.int32: ir.DataType.INT32,
     torch.int64: ir.DataType.INT64,
@@ -109,8 +111,17 @@ def torch_dtype_to_onnx_dtype(dtype: torch.dtype) -> ir.DataType:
 class TorchTensor(ir.Tensor):
     def __init__(self, tensor: torch.Tensor, name: str | None = None):
         # Pass the tensor as the raw data to ir.Tensor's constructor
+        if tensor.dtype == torch.float4_e2m1fn_x2:
+            # Change the shape to the unpacked shape
+            shape = ir.Shape(_type_casting.get_float4_shape(tensor), frozen=True)
+        else:
+            # The base class will set the shape to the tensor's shape
+            shape = None
         super().__init__(
-            tensor, dtype=torch_dtype_to_onnx_dtype(tensor.dtype), name=name
+            tensor,
+            dtype=torch_dtype_to_onnx_dtype(tensor.dtype),
+            shape=shape,
+            name=name,
         )
 
     def numpy(self) -> npt.NDArray:
@@ -132,6 +143,10 @@ class TorchTensor(ir.Tensor):
             ir.DataType.FLOAT8E5M2FNUZ,
         }:
             return self.raw.view(torch.uint8).numpy(force=True).view(self.dtype.numpy())
+        if self.dtype == ir.DataType.FLOAT4E2M1:
+            return _type_casting.unpack_float4x2_as_uint8(self.raw).view(
+                self.dtype.numpy()
+            )
 
         return self.raw.numpy(force=True)
 
@@ -213,7 +228,13 @@ def _set_shape_type(
         logger.warning("Setting shape and type of tensors is not supported yet")
     if isinstance(meta_val, torch.Tensor):
         dims = []
-        for dim in meta_val.shape:
+        shape: tuple[int, ...]
+        if meta_val.dtype == torch.float4_e2m1fn_x2:
+            # Change the shape to the unpacked shape
+            shape = _type_casting.get_float4_shape(meta_val)
+        else:
+            shape = meta_val.shape
+        for dim in shape:
             if isinstance(dim, int):
                 dims.append(dim)
             else:

torch/onnx/_internal/exporter/_dispatching.py

@@ -27,6 +27,7 @@ _TORCH_DTYPE_TO_ONNX_COMPATIBLE: dict[torch.dtype, ir.DataType] = {
     torch.float8_e4m3fnuz: ir.DataType.FLOAT8E4M3FNUZ,
     torch.float8_e5m2: ir.DataType.FLOAT8E5M2,
     torch.float8_e5m2fnuz: ir.DataType.FLOAT8E5M2FNUZ,
+    torch.float4_e2m1fn_x2: ir.DataType.FLOAT4E2M1,
     torch.int16: ir.DataType.INT16,
     torch.int32: ir.DataType.INT32,
     torch.int64: ir.DataType.INT64,
@@ -95,6 +96,7 @@ def _param_type_compatible_with_arg(
         ir.TensorType(ir.DataType.INT32),
         ir.TensorType(ir.DataType.INT64),
         # Int inputs can be casted to a float too
+        ir.TensorType(ir.DataType.FLOAT4E2M1),
         ir.TensorType(ir.DataType.FLOAT8E4M3FN),
         ir.TensorType(ir.DataType.FLOAT8E4M3FNUZ),
         ir.TensorType(ir.DataType.FLOAT8E5M2),
@@ -105,6 +107,7 @@ def _param_type_compatible_with_arg(
     }:
         return True
     if isinstance(value, float) and param.type_constraint.allowed_types & {
+        ir.TensorType(ir.DataType.FLOAT4E2M1),
         ir.TensorType(ir.DataType.FLOAT8E4M3FN),
         ir.TensorType(ir.DataType.FLOAT8E4M3FNUZ),
         ir.TensorType(ir.DataType.FLOAT8E5M2),

torch/onnx/_internal/exporter/_type_casting.py

@@ -0,0 +1,32 @@
+import numpy as np
+
+import torch
+
+
+def unpack_float4x2_as_uint8(tensor: torch.Tensor) -> np.ndarray:
+    """Convert a float4x2 tensor to unpacked uint8 np array."""
+    assert tensor.dtype == torch.float4_e2m1fn_x2
+    data = tensor.view(torch.uint8).numpy(force=True).flatten()
+    result_size = tensor.numel() * 2
+    result = np.empty([result_size], dtype=np.uint8)
+    array_low = data & np.uint8(0x0F)
+    array_high = data & np.uint8(0xF0)
+    array_high >>= np.uint8(4)
+    result[0::2] = array_low
+    result[1::2] = array_high
+    result.resize(get_float4_shape(tensor), refcheck=False)
+    return result
+
+
+def get_float4_shape(tensor: torch.Tensor) -> tuple[int, ...]:
+    """Get the shape of an unpacked float4 tensor.
+
+    The float4_e2m1fn_x2 type is a shell type described in
+    https://github.com/pytorch/pytorch/issues/146414.
+
+    the shell dtype is takes up 1 byte per element and semantically represents
+    two fp4 values packed into 1 byte. Semantically it represents (*tensor.shape, 2)
+    fp4 elements.
+    """
+    assert tensor.dtype == torch.float4_e2m1fn_x2
+    return (*tensor.shape, 2)

torch/onnx/ops/__init__.py

@@ -38,6 +38,9 @@ _TORCH_DTYPE_TO_ONNX_DTYPE = {
     torch.float8_e4m3fnuz: 18,  # FLOAT8E4M3FNUZ
     torch.float8_e5m2: 19,  # FLOAT8E5M2
     torch.float8_e5m2fnuz: 20,  # FLOAT8E5M2FNUZ
+    # 21 = UINT4
+    # 22 = INT4
+    torch.float4_e2m1fn_x2: 23,  # FLOAT4E2M1
 }
 
 

torch/onnx/ops/_symbolic_impl.py

@@ -40,7 +40,7 @@ _ONNX_DTYPE_TO_TORCH_DTYPE: dict[int, torch.dtype] = {
     20: torch.float8_e5m2fnuz,  # FLOAT8E5M2FNUZ
     21: torch.uint8,  # UINT4
     22: torch.uint8,  # INT4
-    23: torch.uint8,  # FLOAT4E2M1
+    23: torch.float4_e2m1fn_x2,  # FLOAT4E2M1
 }
 
 _INT_TYPE = "i"