[Gradient Compression] Refactor CommHookInterface and PythonCommHook. (#46512)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46512
1. Merge 1-line PythonCommHook constructor into the header for simplicity.
2. Move the implementation of PythonCommHook destructor from the header file to cpp file.
3. Rename processFuture method as parseHookResult for readability.
4. Simplify some comments.
Original PR issue: C++ DDP Communication Hook https://github.com/pytorch/pytorch/issues/46348
ghstack-source-id: 115161086
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_ddp_comm_hook_sparse_gradients
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_ddp_comm_hook_allreduce_with_then_hook_nccl
buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_ddp_comm_hook_future_passing_gpu_gloo
Reviewed By: jiayisuse
Differential Revision: D24374282
fbshipit-source-id: c8dbdd764bca5b3fa247708f1218cb5ff3e321bb
