Add an API to set the number of threads in C10 thread pool (#16669)
Summary:
Tested locally on machine translation service
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16669
Differential Revision: D13927858
Pulled By: jamesr66a
fbshipit-source-id: efcb8c21e0c2f76ac37967e6f52967da515595c3
