[Checkpoint][2D][3/N] Add nested_tensors for distributed checkpoint to core distributed (#89501)
This PR moves nested_tensors to torch.distributed.checkpoint. This is a pre-req for enabling 2D checkpoint.
This flattens sharded tensors in state_dict. It is used when saving and loading FSDP SHARDED_STATE_DICT.
Docstring, individual and integration test will be added in the following PRs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89501
Approved by: https://github.com/wanchaol
