Fix compatibility with Python <= 3.8 (#172)

Summary: Pull Request resolved: #172

Reviewed By: JKSenthil

Differential Revision: D55627601

fbshipit-source-id: a071b20ed363b617bd50d88620f78e76cb0a7214

torchsnapshot/io_preparers/sharded_tensor.py

@@ -17,7 +17,7 @@
 from dataclasses import dataclass
 from functools import reduce
 from operator import mul
-from typing import Callable, List, Optional, Tuple
+from typing import Callable, List, Optional, Tuple, Union
 
 import torch
 from torch.distributed._shard.sharded_tensor import (
@@ -199,7 +199,7 @@ def prepare_read(
         cls,
         entry: ShardedTensorEntry,
         obj_out: Optional[ShardedTensor] = None,
-    ) -> Tuple[List[ReadReq], Future[ShardedTensor | torch.Tensor]]:
+    ) -> Tuple[List[ReadReq], Future[Union[ShardedTensor, torch.Tensor]]]:
         # Note: in case obj_out is None, a Future[Tensor] will be returned
         if obj_out is None:
             obj_out = ShardedTensorIOPreparer.empty_tensor_from_sharded_tensor_entry(