Fix store based barrier to only use 'add'. (#49930)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49930
Certain store implementations don't work well when we use get() and
add() on the same key. To avoid this issue, we only use add() in the store
based barrier. The buggy store implementations can't be properly fixed due to
legacy reasons.
Test Plan:
1) unit tests.
2) waitforbuildbot
Reviewed By: osalpekar
Differential Revision: D25725386
fbshipit-source-id: 1535e2629914de7f78847b730f8764f92cde67e7
