[Reland][Gradient Compression] Apply division first to avoid overflow (#59576)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59576
If the gradients before allreduce are large, then the sum after allreduce may overflow, especially for FP16. Therefore, apply the division before allreduce.
This fix is applied to both C++ and Python comm hooks.
ghstack-source-id: 130754510
Test Plan:
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_ddp_comm_hook_allreduce_hook_nccl
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_default_ddp_comm_hooks_nccl
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_fp16_compress_wrapper_nccl
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_builtin_ddp_comm_hooks_nccl
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_powerSGD_ddp_comm_hook_nccl
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_ddp_comm_hook_allreduce_hook_nccl_grad_is_view
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_default_ddp_comm_hooks_nccl_is_view
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_fp16_compress_wrapper_is_view
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_builtin_ddp_comm_hooks_nccl_grad_is_view
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_powerSGD_ddp_comm_hook_nccl_grad_is_view
Reviewed By: rohan-varma
Differential Revision: D28941327
fbshipit-source-id: 932e8ddbdb2bfd609a78943f6dc390d3d6ca333f
