Use global lock instead of per instance lock. (#31404)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31404
Multiple "trainers" could each create different instances of DistributedOptimizer, which means we can still have a race condition unless we do a trully global per worker lock.
ghstack-source-id: 95874624
Test Plan: run unit tests -- unfortunatelly due to the non-deterministic behavior it's not clear how to unit test this properly.
Differential Revision: D19154248
fbshipit-source-id: fab6286c17212f534f1bd1cbdf9f0de002d48c74
