Move python-independent c10d implementations to torch/lib (#47309)
Summary:
* This is a pre-step to build c10d into libtorch
* Includes a minor cleanup in c10d/CMakeLists.txt
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47309
Reviewed By: wanchaol
Differential Revision: D24711768
Pulled By: gmagogsfm
fbshipit-source-id: 6f9e0a6a73c30f5ac7dafde9082efcc4b725dde1
