Revert D27652485: [nnc] Enable CPU fusion only when num_threads == 1
Test Plan: revert-hammer
Differential Revision:
D27652485 (https://github.com/pytorch/pytorch/commit/e7e164f9e67d100a192f6f29709b9331b4265b1f)
Original commit changeset: 182580cf758d
fbshipit-source-id: e3c95b06d1eef668095f3cf461485395179d94af
