Make check_feature_gate_key PT2 compatible (pytorch#3426)

Summary:

X-link: facebookresearch/FBGEMM#513

Add a new API for `check_feature_gate_key` that is PT2 compatible.
PT2 complains when an op does not take/return a tensor.  Thus,
`check_feature_gate_key_pt2` (the new API) takes a dummy tensor as an
input and returns a boolean tensor as an output.

Differential Revision: D66611785

fbgemm_gpu/fbgemm_gpu/config/feature_list.py

@@ -71,6 +71,10 @@ class FeatureGate:
         FeatureGate.is_enabled(FeatureGateName.TBE_V2)
     """
 
+    dummy_tensor = torch.empty(1)
+
     @classmethod
     def is_enabled(cls, feature: FeatureGateName) -> bool:
-        return torch.ops.fbgemm.check_feature_gate_key(feature.name)
+        return torch.ops.fbgemm.check_feature_gate_key_pt2(
+            cls.dummy_tensor, feature.name
+        )[0].item()