Fix Pyre test on OSS (#2842)

Summary:
Pull Request resolved: #2842

Fixing pyre error:
````
torchrec/ir/utils.py:178:4 Incompatible return type [7]: Expected `DIM` but got `Dim`.

Reviewed By: TroyGarden

Differential Revision: D71656776

fbshipit-source-id: 3eb552d33c3be3e3fe54e2d55ad043d750cb2cc7

Author: aporialiao

torchrec/ir/utils.py

@@ -175,6 +175,7 @@ def _get_dim(name: str, min: Optional[int] = None, max: Optional[int] = None) ->
     """
     dim = f"{name}_{DYNAMIC_DIMS[name]}"
     DYNAMIC_DIMS[name] += 1
+    # pyre-ignore[7]: Expected `DIM` but got `Dim`.
     return Dim(dim, min=min, max=max)