Allow override of backend in dist.new_group() (#18595)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18595
There is no need to force the backend to be the same as the global
process group, as long as the backend is "nccl" or "gloo".
Reviewed By: mrshenli
Differential Revision: D14657204
fbshipit-source-id: 868817b9f219e3be8db0761a487f0027ed46663b
