[PT/ShardedTensor]Allow zero size local shard (#65007)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65007
Relax shard size check in ShardMetadata to allow zero size local shard.
When sharding a tensor on N ranks, some ranks may have empty shard allocated. As we are assuming SPMD, the ranks w/ empty shard still need to participate in all collectives, and we need to allow this in ShardMetadata.
Test Plan: Unit tests and CLI
Reviewed By: jiaqizhai, wanchaol
Differential Revision: D30926566
fbshipit-source-id: afa562c94ffa8f8d91d65ddb4c348156d871dc36
