[pthreadpool] Cap max thread count to fix TSAN issues (#83950)
Summary: Cap the thread count to 64 unconditionally to solve this tsan issue which leads to harder to debug, flaky test failures.
Test Plan: CI
Reviewed By: kimishpatel
Differential Revision: D38136212
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83950
Approved by: https://github.com/kimishpatel
