[DCP] Modify tensor saving logic in DCP (#106415)
Currently, DCP treats tensors as duplicates and only saves them on rank0. This won't work for PiPPy as PiPPy does have unique tensors across different ranks. With the current setup, we would only be saving the tensors on rank0 (coordinator rank).
In this PR, we are changing to letting each rank create its own WriteItem for tensors. For the ones that does replicate across different ranks, we are handling it thru dedup_tensors(), which will dedup the replicate WriteItem so we only do the actual writing once.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106415
Approved by: https://github.com/wz337
