Add fake impl for aten.unique_dim (#126561)

Follow-up to #113118 and #124306.

Developed in coordination with the solution to microsoft/onnxscript#1547

This PR adds the missing fake tensor implementation for `aten.unique_dim`, thus enabling tracing and compilation of `torch.unique` when `dim` is not None.

Local testing has proceeded with the following simple script (provided that one has checked out the changes in microsoft/onnxscript#1547):

```python
    import onnx
    import onnxruntime as ort
    import logging
    import numpy as np
    onnx_program = torch.onnx.dynamo_export(
        lambda x: torch.unique(x,
                               dim=0,
                               return_inverse=True),
        torch.arange(10),
        export_options=torch.onnx.ExportOptions(
            dynamic_shapes=True,
            diagnostic_options=torch.onnx.DiagnosticOptions(
                verbosity_level=logging.DEBUG)))
    onnx_program.save("torch_unique.onnx")
    onnx_inputs = onnx_program.adapt_torch_inputs_to_onnx(torch.arange(10))
    onnx_outputs = onnx_program(*onnx_inputs)
    loaded_onnx_program = onnx.load("torch_unique.onnx")
    onnx.checker.check_model(loaded_onnx_program)
    ort_session = ort.InferenceSession("torch_unique.onnx")
    inputs = np.random.randint(0, 10, 10)
    print(f"Inputs: {inputs}")
    outputs = ort_session.run(None,
                              {
                                  "l_x_": inputs
                              })
    print(f"Outputs: {outputs}")
    print("Success")
```

Co-authored-by: Edward Z. Yang <[email protected]>
Pull Request resolved: #126561
Approved by: https://github.com/ezyang

test/test_ops.py

@@ -2522,8 +2522,8 @@ def map_to_fake(e):
                         or name in sometimes_dynamic_output_op_test
                     )
                     self.assertTrue(
-                        mode.shape_env is None
-                        or not mode.shape_env.allow_dynamic_output_shape_ops
+                        fake_mode.shape_env is None
+                        or not fake_mode.shape_env.allow_dynamic_output_shape_ops
                         or name not in supported_dynamic_output_op_tests
                     )
                 except torch._subclasses.fake_tensor.DataDependentOutputException:

test/test_proxy_tensor.py

@@ -2003,7 +2003,6 @@ def f(t):
     xfail('nn.functional.ctc_loss'),  # aten._ctc_loss.Tensor - couldn't find symbolic meta function/decomposition
     xfail('quantile', ''),  # Could not run 'aten::equal' with arguments from the 'Meta' backend.
     xfail('unique_consecutive', ''),  # aten.unique_consecutive.default - couldn't find symbolic meta function/decomposition
-    xfail('unique', ''),  # aten._unique2.default - couldn't find symbolic meta function/decomposition
 
     xfail('max_pool2d_with_indices_backward', ''),  # Expected a value of type 'List[int]' for argument 'kernel_size' but...
 
@@ -2034,8 +2033,6 @@ def f(t):
 inplace_symbolic_tensor_failures = {
     # bugs
     xfail('float_power', ''),  # base given to float_power_ has dtype Float but the operation's result requires dtype Double
-    # decomp not implemented
-    xfail('unique', ''),
 }
 
 out_symbolic_tensor_failures = {

torch/_subclasses/fake_impls.py

@@ -258,9 +258,8 @@ def dyn_shape(fake_mode, func, *args, **kwargs):
     raise DynamicOutputShapeException(func)
 
 
-@register_op_impl(aten._unique2.default)
-def unique2(
-    fake_mode, func, arg, sorted=True, return_inverse=False, return_counts=False
+def _unique(
+    fake_mode, func, arg, dim, sorted=True, return_inverse=False, return_counts=False
 ):
     if (
         fake_mode.shape_env is None
@@ -269,7 +268,8 @@ def unique2(
         # Without symints/symfloats, cannot handle this
         raise DynamicOutputShapeException(func)
 
-    if (nnz := arg.unique_memo) is None:
+    # Do not use a memo for unique_dim
+    if dim is not None or (nnz := arg.unique_memo) is None:
         # Avoid importing sympy at a module level
         from torch.fx.experimental.symbolic_shapes import (
             _constrain_range_for_size,
@@ -291,28 +291,59 @@ def unique2(
 
             maxval = sys.maxsize - 1
 
-            if not has_free_symbols(arg.numel()):
-                maxval = int(arg.numel())
+            numel = arg.numel() if dim is None else arg.size(dim)
+            if not has_free_symbols(numel):
+                maxval = int(numel)
 
             _constrain_range_for_size(nnz, max=maxval)
 
-        arg.unique_memo = nnz
+        if dim is None:
+            arg.unique_memo = nnz
 
-    ret = [arg.new_empty((nnz,))]
+    if dim is None:
+        ret = [arg.new_empty((nnz,))]
+    else:
+        ret = [arg.new_empty(*arg.shape[:dim], nnz, *arg.shape[dim + 1 :])]
 
-    if return_inverse:
-        ret.append(torch.empty_like(arg))
+    return_if_dim_and_cpu = dim is not None and arg.fake_device == torch.device("cpu")
+    if return_inverse or return_if_dim_and_cpu:
+        inverse = arg.new_empty(arg.shape if dim is None else (arg.shape[dim],))
     else:
-        ret.append(arg.new_empty(0))
+        inverse = arg.new_empty(0)
+    ret.append(inverse)
 
-    if return_counts:
-        ret.append(torch.empty_like(arg))
+    if return_counts or return_if_dim_and_cpu:
+        counts = arg.new_empty(ret[0].shape if dim is None else (ret[0].shape[dim],))
     else:
-        ret.append(arg.new_empty(0))
+        counts = arg.new_empty(0)
+    ret.append(counts)
 
     return tuple(ret)
 
 
+@register_op_impl(aten._unique2.default)
+def unique2(
+    fake_mode, func, arg, sorted=True, return_inverse=False, return_counts=False
+):
+    return _unique(fake_mode, func, arg, None, sorted, return_inverse, return_counts)
+
+
+@register_op_impl(aten.unique_dim.default)
+def unique_dim(
+    fake_mode, func, arg, dim, sorted=True, return_inverse=False, return_counts=False
+):
+    return _unique(
+        fake_mode,
+        func,
+        arg,
+        # normalize dim to be non-negative
+        dim if dim >= 0 else dim % max(arg.ndim, 1),
+        sorted,
+        return_inverse,
+        return_counts,
+    )
+
+
 @register_op_impl(aten.repeat_interleave.Tensor)
 def repeat_interleave_tensor(fake_mode, func, repeats, output_size=None):
     if output_size is None: