refactor _Dim into Dim

Summary: forward fix T218515233

Differential Revision: D71769231

Author: pianpwk

backends/xnnpack/test/tester/tester.py

@@ -496,14 +496,14 @@ def generate_random_inputs(self):
                         # the function to compute the derived dim.
                         dim_spec = dim_spec.root
                         fn = dim_spec.fn
-                    elif isinstance(dim_spec, torch.export.dynamic_shapes._Dim):
+                    elif isinstance(dim_spec, torch.export.dynamic_shapes.Dim):
                         # Not derived dim so fn is just itself
                         def fn(x):
                             return x
 
                     else:
                         raise RuntimeError(
-                            f"Expected Dynamic Dims to be of type _DerivedDim or _Dim but got {type(dim_spec)}"
+                            f"Expected Dynamic Dims to be of type _DerivedDim or Dim but got {type(dim_spec)}"
                         )
                     dim_name = dim_spec.__name__
                     if dim_name not in dim_name_to_size: