[sharded_tensor] simplify init_from_local_shards API (#64481)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64481
This simplifies `init_from_local_shards` API in sharded tensor, to only require user pass in a list of `Shard` and `overall_size`, instead of ShardedTensorMetadata. We will do the all_gather inside to form a valid ShardedTensorMetadata instead.
TODO: add more test cases to improve coverage.
ghstack-source-id: 141742350
Test Plan: TestShardedTensorFromLocalShards
Reviewed By: pritamdamania87
Differential Revision: D30748504
fbshipit-source-id: 6e97d95ffafde6b5f3970e2c2ba33b76cabd8d8a
