DistributedOptimizer: wait for all workers to finish _LocalOptimizer constructor (#30062)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30062
This allows to catch exceptions during optimizer creation.
ghstack-source-id: 94232436
Test Plan: new unit test.
Differential Revision: D18586108
fbshipit-source-id: 71cfdf337fe803dbea8787b4c68e5a52b70a1f68
