[DDP Communication Hook] Renaming C++ calls to match python API closer (#62735)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62735
Renamed the following
1. getTensor -> getBuffer
2. getTensorRef -> getBufferRef
3. setTensor -> setBuffer
and all associated private variables as well
Reviewed By: SciPioneer
Differential Revision: D30069124
fbshipit-source-id: fa8f1f8a7f3255e6242973bc37b3f7b2731af55d
