[PT-D][Checkpointing] Move distributed checkpointing from torch.distributed._shard.checkpoint to torch.distributed.checkpoint (#88698)
Context in RFC: https://github.com/pytorch/pytorch/issues/86620
.rst file will be finalized in subsequent PRs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88698
Approved by: https://github.com/wanchaol
