[_shard] Add ShardedTensorBase (#82291)
This PR added ShardedTensorBase, which is the base class of
ShardedTensor, and only contains local shards, ShardedTensorMetadata,
and does not have any communication backend attached (i.e ProcessGroup)
Differential Revision: [D38190272](https://our.internmc.facebook.com/intern/diff/D38190272)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82291
Approved by: https://github.com/fduwjj
