Cleanup ProcessGroup.cpp (#69706)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69706
Mostly code modernization, also do not capture unused `this` in
end_handler functor
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D32997009
Pulled By: malfet
fbshipit-source-id: ac907f0c6889ad06d4fb0171964cb05133e5e610
