Smaller shards for sharded tensors (#20)

Summary:
google cloud storage seems to prefer files closer to 100 MB than 1 GB and has less errors like googleapis/google-cloud-ruby#2516

`BrokenPipeError` is not caught by `google-cloud-storage`'s native retry capabilities.

Pull Request resolved: #20

Reviewed By: edward-io

Differential Revision: D37273001

Pulled By: yifuwang

fbshipit-source-id: d7bbf8c796fcdc3d2afea558295bf449cac08682

Author: rllin

torchsnapshot/io_preparer.py

@@ -35,7 +35,7 @@
 
 
 class ShardedTensorIOPreparer:
-    DEFAULT_MAX_SHARD_SIZE_BYTES: int = 1 * 1024 * 1024 * 1024
+    DEFAULT_MAX_SHARD_SIZE_BYTES: int = 512 * 1024 * 1024
 
     @staticmethod
     def subdivide_shard(