Open registration for c10 thread pool (#17788)
Summary:
1. Move ATen threadpool & open registration mechanism to C10
2. Move the `global_work_queue` to use this open registration mechanism, to allow users to substitute in their own
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17788
Reviewed By: zdevito
Differential Revision: D14379707
Pulled By: jamesr66a
fbshipit-source-id: 949662d0024875abf09907d97db927f160c54d45
