[sharded_tensor] add default fields to ShardedTensorMetadata (#62867)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62867
This add default fields for ShardedTensorMetadata, to allow easy construction and modification afterwards.
ghstack-source-id: 135284133
Test Plan: ShardedTensorMetadata validity should be guarded with `init_from_local_shards` API and its tests.
Reviewed By: pritamdamania87
Differential Revision: D30148481
fbshipit-source-id: 0d99f41f23dbeb4201a36109556ba23b9a6c6fb1
