Revert D30417127: Remove flag to toggle CPU fusion in the presence of parallelism
Test Plan: revert-hammer
Differential Revision:
D30417127 (https://github.com/pytorch/pytorch/commit/6600bc96517269c608ea47b76b6bda9476c7bcef)
Original commit changeset: b77d7c68364f
fbshipit-source-id: 6b52fb83a84fe241945e3cb3eeb71050d1d9c8f1
