[Gradient Compression] Allow PowerSGD to run vallina allreduce for the first K iterations (#50973)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50973
This can extend the original PowerSGD method to a hybrid approach: vanilla allreduce + PowerSGD. This can help further improve the accuracy, at the cost of a lower speedup.
Also add more comments on the fields in `PowerSGDState`.
Original PR issue: Investigate Applying PowerSGD to Communication Hook for Gradient Compression #47202
ghstack-source-id: 120257202
Test Plan:
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_powerSGD_ddp_comm_hook_nccl
buck test mode/dev-nosan caffe2/test/distributed:distributed_nccl_fork -- test_DistributedDataParallel_powerSGD_ddp_comm_hook
Reviewed By: rohan-varma
Differential Revision: D26031478
fbshipit-source-id: d72e70bb28ba018f53223c2a4345306980b3084e
