Revert D21232894: Unify PyTorch mobile's threadpool usage.
Test Plan: revert-hammer
Differential Revision:
D21232894 (https://github.com/pytorch/pytorch/commit/b9d3869df357038f798eef579fe1c69cf246887d)
Original commit changeset: 8b3de86247fb
fbshipit-source-id: e6517cfec08f7dd0f4f8877dab62acf1d65afacd
