[Gradient Compression] Surface C++ comm hooks to Python API as built-in comm hooks (#46959)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46959
1. Implemented the Pybind part.
2. In the reducer, once the builtin_comm_hook_type is set, a c++ comm hook instance will be created in Reducer::autograd_hook.
3. Added unit tests for the builit-in comm hooks.
Original PR issue: C++ DDP Communication Hook https://github.com/pytorch/pytorch/issues/46348
ghstack-source-id: 115629230
Test Plan: buck test mode/dev-nosan caffe2/test/distributed:c10d -- test_builtin_ddp_comm_hooks_nccl
Reviewed By: pritamdamania87
Differential Revision: D24471910
fbshipit-source-id: f96b752298549ea2067e2568189f1b394abcd99a
