Add torch.unflatten and improve its docs (#81399)

unflatten now has a free function version in torch.flatten in addition to
    the method in torch.Tensor.flatten.

    Updated docs to reflect this and polished them a little.
    For consistency, changed the signature of the int version of unflatten in
    native_functions.yaml.

    Some override tests were failing because unflatten has unusual
    characteristics in terms of the .int and .Dimname versions having
    different number of arguments so this required some changes
    to test/test_override.py

    Removed support for using mix of integer and string arguments
    when specifying dimensions in unflatten.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81399
Approved by: https://github.com/Lezcano, https://github.com/ngimel

aten/src/ATen/native/TensorShape.cpp

@@ -2899,7 +2899,7 @@ static inline void handle_unflatten_exception(const std::runtime_error &e,
   }
 }
 
-Tensor unflatten(const Tensor& self, int64_t dim, IntArrayRef sizes, c10::optional<DimnameList> names) {
+Tensor unflatten_impl(const Tensor& self, int64_t dim, IntArrayRef sizes, c10::optional<DimnameList> names) {
   dim = maybe_wrap_dim(dim, self.dim());
 
   TORCH_CHECK(sizes.size() > 0, "unflatten: sizes must be non-empty");
@@ -2938,8 +2938,12 @@ Tensor unflatten(const Tensor& self, int64_t dim, IntArrayRef sizes, c10::option
   return result;
 }
 
+Tensor unflatten(const Tensor& self, int64_t dim, IntArrayRef sizes) {
+  return native::unflatten_impl(self, dim, sizes, c10::nullopt);
+}
+
 Tensor unflatten(const Tensor& self, Dimname dim, IntArrayRef sizes, DimnameList names) {
-  return native::unflatten(self, dimname_to_position(self, dim), sizes, names);
+  return native::unflatten_impl(self, dimname_to_position(self, dim), sizes, names);
 }
 
 Tensor view_as(const Tensor& self, const Tensor& other) {

aten/src/ATen/native/native_functions.yaml

@@ -2247,11 +2247,11 @@
 - func: flatten.DimnameList(Tensor(a) self, Dimname[] dims, Dimname out_dim) -> Tensor(a)
   variants: function, method
 
-- func: unflatten.int(Tensor(a) self, int dim, int[] sizes, Dimname[]? names=None) -> Tensor(a)
+- func: unflatten.int(Tensor(a) self, int dim, int[] sizes) -> Tensor(a)
-  variants: method
+  variants: function, method
 
 - func: unflatten.Dimname(Tensor(a) self, Dimname dim, int[] sizes, Dimname[] names) -> Tensor(a)
-  variants: method
+  variants: function, method
 
 - func: fill.Scalar(Tensor self, Scalar value) -> Tensor
   variants: function

docs/source/named_tensor.rst

@@ -300,7 +300,6 @@ operators, see :ref:`name_inference_reference-doc`.
    .. automethod:: align_as
    .. automethod:: align_to
 
-   .. automethod:: unflatten
    .. py:method:: flatten(dims, out_dim) -> Tensor
       :noindex:
 

docs/source/tensors.rst

@@ -685,6 +685,7 @@ Tensor class reference
     Tensor.type
     Tensor.type_as
     Tensor.unbind
+    Tensor.unflatten
     Tensor.unfold
     Tensor.uniform_
     Tensor.unique

docs/source/torch.rst

@@ -533,6 +533,7 @@ Other Operations
     tril_indices
     triu
     triu_indices
+    unflatten
     vander
     view_as_real
     view_as_complex

test/forward_backward_compatibility/check_forward_backward_compatibility.py

@@ -114,6 +114,7 @@ ALLOW_LIST = [
     ("c10d::broadcast", datetime.date(2022, 6, 25)),
     ("aten::.*functional", datetime.date(2022, 8, 1)),
     ("aten::_foreach.*", datetime.date(2022, 8, 1)),
+    ("aten::unflatten", datetime.date(2022, 8, 10)),
     # TODO: FIXME: prims shouldn't be checked
     ("prims::.*", datetime.date(9999, 1, 1)),
 ]

test/test_namedtensor.py

@@ -1083,22 +1083,13 @@ class TestNamedTensor(TestCase):
     def test_unflatten(self):
         # test args: tensor, int, namedshape
         self.assertTrue(torch.equal(
-            torch.ones(4).unflatten(0, (('A', 2), ('B', 2))),
+            torch.ones(4, names=('A',)).unflatten('A', (('A', 2), ('B', 2))),
             torch.ones(2, 2, names=('A', 'B'))))
         self.assertTrue(torch.equal(
-            torch.ones(4).unflatten(0, [('A', 2), ('B', 2)]),
+            torch.ones(4, names=('A',)).unflatten('A', [('A', 2), ('B', 2)]),
             torch.ones(2, 2, names=('A', 'B'))))
         self.assertTrue(torch.equal(
-            torch.ones(4).unflatten(0, (['A', 2], ['B', 2])),
+            torch.ones(4, names=('A',)).unflatten('A', (['A', 2], ['B', 2])),
-            torch.ones(2, 2, names=('A', 'B'))))
-        self.assertTrue(torch.equal(
-            torch.ones(4).unflatten(-1, (['A', 2], ['B', 2])),
-            torch.ones(2, 2, names=('A', 'B'))))
-        self.assertTrue(torch.equal(
-            torch.ones(4).unflatten(-1, (['A', -1], ['B', 2])),
-            torch.ones(2, 2, names=('A', 'B'))))
-        self.assertTrue(torch.equal(
-            torch.ones(4).unflatten(-1, (['A', 2], ['B', -1])),
             torch.ones(2, 2, names=('A', 'B'))))
         self.assertTrue(torch.equal(
             torch.ones(2, 10, names=('A', 'B')).unflatten('B', (['B1', -1],)),
@@ -1112,18 +1103,13 @@ class TestNamedTensor(TestCase):
                  .unflatten('B', (['B1', 3], ['B2', -1], ['B3', 4])),
             torch.ones(2, 3, 0, 4, names=('A', 'B1', 'B2', 'B3'))))
 
-        # test args: namedtensor, int, namedshape
-        self.assertTrue(torch.equal(
-            torch.ones(2, 4, names=('A', 'B')).unflatten(1, (('B1', 2), ('B2', 2))),
-            torch.ones(2, 2, 2, names=('A', 'B1', 'B2'))))
-
         # test args: namedtensor, str, namedshape
         self.assertTrue(torch.equal(
             torch.ones(2, 4, names=('A', 'B')).unflatten('B', (('B1', 2), ('B2', 2))),
             torch.ones(2, 2, 2, names=('A', 'B1', 'B2'))))
 
         # test invalid args: namedtensor, str, sizes
-        with self.assertRaisesRegex(TypeError, r"received an invalid combination of arguments"):
+        with self.assertRaisesRegex(TypeError, r"unflatten\(\): argument 'dim' \(position 1\) must be int, not str"):
             torch.tensor([1], names=('A',)).unflatten('A', (1, 1))
 
         # test invalid args: namedtensor, int, sizes

test/test_overrides.py

@@ -337,7 +337,7 @@ def generate_tensor_like_torch_implementations():
     msg = (
         "The following functions are not tested for __torch_function__ "
         "support, please ensure there is an entry in the dict returned by "
-        "torch._overrides.get_testing_overrides for this function or if a "
+        "torch.overrides.get_testing_overrides for this function or if a "
         "__torch_function__ override does not make sense, add an entry to "
         "the tuple returned by torch._overrides.get_ignored_functions.\n\n{}"
     )
@@ -648,7 +648,11 @@ def generate_tensor_like_override_tests(cls):
                     func_args.append(3.5)
                 elif t == 'bool':
                     func_args.append(False)
-                elif t.startswith('int') or t in {'Dimname', 'DimnameList'}:
+                elif t == 'Dimname':
+                    func_args.append("")
+                elif t == 'DimnameList':
+                    func_args.append([""])
+                elif t.startswith('int'):
                     func_args.append(0)
                 elif t in {'Stream'}:
                     func_args.append(torch.Stream())

test/test_torch.py

@@ -5857,7 +5857,7 @@ class TestTorch(TestCase):
                          torch.ones(2, 3, 0, 4, 5, 2))
 
         # test invalid args: tensor, str, sizes
-        with self.assertRaisesRegex(TypeError, r"received an invalid combination of arguments"):
+        with self.assertRaisesRegex(TypeError, r"unflatten\(\): argument 'dim' \(position 1\) must be int, not str"):
             torch.tensor([1]).unflatten('A', (1, 1))
 
         # test invalid args: tensor, str, namedshape

torch/_tensor.py

@@ -1135,34 +1135,10 @@ class Tensor(torch._C._TensorBase):
         )
 
     def unflatten(self, dim, sizes):
-        r"""Expands the dimension :attr:`dim` of the :attr:`self` tensor over multiple dimensions
+        r"""
-        of sizes given by :attr:`sizes`.
+        unflatten(dim, sizes) -> Tensor
 
-        * :attr:`sizes` is the new shape of the unflattened dimension and it can be a `Tuple[int]` as well
+        See :func:`torch.unflatten`.
-          as `torch.Size` if :attr:`self` is a `Tensor`, or `namedshape` (Tuple[(name: str, size: int)])
-          if :attr:`self` is a `NamedTensor`. The total number of elements in sizes must match the number
-          of elements in the original dim being unflattened.
-
-        Args:
-            dim (Union[int, str]): Dimension to unflatten
-            sizes (Union[Tuple[int] or torch.Size, Tuple[Tuple[str, int]]]): New shape of the unflattened dimension
-
-        Examples:
-            >>> torch.randn(3, 4, 1).unflatten(1, (2, 2)).shape
-            torch.Size([3, 2, 2, 1])
-            >>> torch.randn(3, 4, 1).unflatten(1, (-1, 2)).shape # the size -1 is inferred from the size of dimension 1
-            torch.Size([3, 2, 2, 1])
-            >>> torch.randn(2, 4, names=('A', 'B')).unflatten('B', (('B1', 2), ('B2', 2)))
-            tensor([[[-1.1772,  0.0180],
-                    [ 0.2412,  0.1431]],
-                    [[-1.1819, -0.8899],
-                    [ 1.5813,  0.2274]]], names=('A', 'B1', 'B2'))
-            >>> torch.randn(2, names=('A',)).unflatten('A', (('B1', -1), ('B2', 1)))
-            tensor([[-0.8591],
-                    [ 0.3100]], names=('B1', 'B2'))
-
-        .. warning::
-            The named tensor API is experimental and subject to change.
 
         """
         if has_torch_function_unary(self):
@@ -1177,6 +1153,8 @@ class Tensor(torch._C._TensorBase):
         ):
             names, sizes = unzip_namedshape(sizes)
             return super(Tensor, self).unflatten(dim, sizes, names)
+        else:
+            return super(Tensor, self).unflatten(dim, sizes)
 
     def rename_(self, *names, **rename_map):
         """In-place version of :meth:`~Tensor.rename`."""

torch/_torch_docs.py

@@ -4552,6 +4552,41 @@ Example::
     ),
 )
 
+add_docstr(
+    torch.unflatten,
+    r"""
+unflatten(input, dim, sizes) -> Tensor
+
+Expands a dimension of the input tensor over multiple dimensions.
+
+.. seealso::
+
+    :func:`torch.flatten` the inverse of this function. It coalesces several dimensions into one.
+
+Args:
+    {input}
+    dim (int): Dimension to be unflattened, specified as an index into
+         ``input.shape``.
+    sizes (Tuple[int]): New shape of the unflattened dimension.
+         One of its elements can be `-1` in which case the corresponding output
+         dimension is inferred. Otherwise, the product of ``sizes`` *must*
+         equal ``input.shape[dim]``.
+
+Returns:
+    A View of input with the specified dimension unflattened.
+
+Examples::
+    >>> torch.unflatten(torch.randn(3, 4, 1), 1, (2, 2)).shape
+    torch.Size([3, 2, 2, 1])
+    >>> torch.unflatten(torch.randn(3, 4, 1), 1, (-1, 2)).shape
+    torch.Size([3, 2, 2, 1])
+    >>> torch.unflatten(torch.randn(5, 12, 3), -1, (2, 2, 3, 1, 1)).shape
+    torch.Size([5, 2, 2, 3, 1, 1, 3])
+""".format(
+        **common_args
+    ),
+)
+
 add_docstr(
     torch.gather,
     r"""

torch/csrc/api/src/nn/modules/linear.cpp

@@ -126,7 +126,7 @@ Tensor UnflattenImpl::forward(const Tensor& input) {
     }
     return input.unflatten(dimname, sizes, names);
   }
-  return input.unflatten(options.dim(), options.sizes(), torch::nullopt);
+  return input.unflatten(options.dim(), options.sizes());
 }
 
 // ============================================================================

torch/overrides.py

@@ -1073,6 +1073,7 @@ def get_testing_overrides() -> Dict[Callable, Callable]:
         torch.true_divide: lambda input, other: -1,
         torch.trunc: lambda input, out=None: -1,
         torch.unbind: lambda input, dim=0: -1,
+        torch.unflatten: lambda input, dim, sizes, names: -1,
         torch.unique: lambda input, sorted=True, return_inverse=False, return_counts=False, dim=None: -1,
         torch.unique_consecutive: lambda input, return_inverse=False, return_counts=False, dim=None: -1,
         torch.unsafe_chunk: lambda input, chunks, dim=0: -1,