Cleanup init_process_group (#19033)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19033
torch.distributed.init_process_group() has had many parameters added, but the contract isn't clear. Adding documentation, asserts, and explicit args should make this clearer to callers and more strictly enforced.
Reviewed By: mrshenli
Differential Revision: D14813070
fbshipit-source-id: 80e4e7123087745bed436eb390887db9d1876042
