[sharded_tensor] refactor sharded_tensor file structure (#67199)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67199
This PR refactors _sharded_tensor package so that it splits from api.py, and add different components to make it more modularized, this will also help us resolve circular dependency due to increasing code size and better organize the package:
* api.py: sharded tensor APIs
* metadata.py: Metadata definition for ShardedTensors
* shard.py: Shard definition for ShardedTensor
* utils.py: utility functions for validation, etc.
ghstack-source-id: 141533618
Test Plan: test_sharded_tensor.py
Reviewed By: pritamdamania87
Differential Revision: D31904249
fbshipit-source-id: c747d96e131a1d4731991ec4ac090f639dcb369b
