[Gradient Compression] Add CppCommHook subclass for supporting the C++ API of communication hook. (#46566)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46566
Only provides an interface. Some built-in implementations will be provided in a follow-up commit.
Original PR issue: C++ DDP Communication Hook https://github.com/pytorch/pytorch/issues/46348
ghstack-source-id: 115319038
Test Plan: waitforbuildbot
Reviewed By: pritamdamania87
Differential Revision: D24379460
fbshipit-source-id: 8382dc4185c7c01d0ac5b3498e1bead785bccec5
