Add pthreadpool_create and pthreadpool_destroy (#15492)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15492
Add pthreadpool_create and pthreadpool_destroy, which are used by NNPACK tests.
Reviewed By: Maratyszcza
Differential Revision: D13540997
fbshipit-source-id: 628c599df87b552ca1a3703854ec170243f04d2e
