[pthreadpool] Apply cap for macos builds (#55435)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55435
We've seen issues from the macos skylight app that PyTorch is super slow due to the lack of cap support in pthreadpools. For mac builds, we set the thread count to `#threads/2`.
ghstack-source-id: 125900852
Test Plan:
- Sandcastle CI
- CircleCI
Reviewed By: kimishpatel
Differential Revision: D27578871
fbshipit-source-id: 7b947bc5d6cf289378abf5f479575e112325d02b
