Make c10::ThreadPool::available_ atomic. (#58457)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58457
This variable had concurrent read/write access without any
synchronization. The issue was caught and reported by TSAN.
ghstack-source-id: 129311384
Test Plan:
1) Verify test locally.
2) waitforbuildbot.
Reviewed By: ezyang
Differential Revision: D28498116
fbshipit-source-id: 89af068467fed64c131d743504c0cecf3017d638
