fix py3.9 build (#2660)

Summary:
Pull Request resolved: #2660

The `|` operator is only supported for python >= 3.10, which breaks python 3.9 build

Reviewed By: dstaay-fb

Differential Revision: D67723223

fbshipit-source-id: a49208aa8d819c3047b75a52ab7ba6cb6c2638b8

Author: sarckk

torchrec/distributed/mc_modules.py

@@ -66,7 +66,9 @@
 
 @dataclass
 class EmbeddingCollectionContext(Multistreamable):
-    sharding_contexts: List[InferSequenceShardingContext | SequenceShardingContext]
+    sharding_contexts: List[
+        Union[InferSequenceShardingContext, SequenceShardingContext]
+    ]
 
     def record_stream(self, stream: torch.Stream) -> None:
         for ctx in self.sharding_contexts: