Make leak_corrupted_threadpool non-atomic (#55341)
Summary:
Following up on https://github.com/pytorch/pytorch/pull/54895#discussion_r606402656.
A race-condition wouldn't arise because `leak_corrupted_threadpool` can be set to true only after fork via the `pthread_atfork` handler, when a (child) process would be single-threaded. It's set to false also when the process is still single-threaded (`pthreadpool` is called during an invocation to `set_num_threads`, prior to which a child process would remain single-threaded). All threads (if & when multiple threads would be created) would always see `leak_corrupted_threadpool` as false if it would be accessed concurrently.
Since no reader threads can exist while a writer thread changes its value (false->true and true->false), `leak_corrupted_threadpool` might as well be a non-atomic bool.
### Pros
1. No thread-synchronization is required for `leak_corrupted_threadpool`, as it's a non-atomic bool.
2. The call to `compare_exchange_strong` has been be removed.
cc: malfet VitalyFedyunin ezyang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55341
Reviewed By: albanD
Differential Revision: D27669442
Pulled By: ezyang
fbshipit-source-id: 926cb5c1b0a537c1c2ab164b0d51d37c1f1b67f0
