Remove `_fork_processes` from common_distributed.py (#63711)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63711
This removes `_fork_process` from common_distributed.py and fixes all
other callpoints to use `spawn_process` instead.
ghstack-source-id: 136395719
Test Plan: waitforbuildbot
Reviewed By: xush6528
Differential Revision: D30463834
fbshipit-source-id: 0c09e8a996d0e5b912c8cdd45488a39951bac4db
