Reland: D27652485: [nnc] Enable CPU fusion only when num_threads == 1" (#56120)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56120
This reverts commit ad17fadbfc786dc1ccb42e822208ff03c2a2b72c (D27786457).
The big annoyance here is that depending on the threading mode you may not be
able to toggle num_threads at will, so the fusion tests won't fail.
I hate this solution, but I'm adding a secondary override for the TE fuser.
Now you need to both turn on fusion (_jit_override_can_fuse_on_cpu), and you're
OK if you're running with 1 thread, or you can add
`_jit_set_texpr_parallel_cpu_enabled` to enable it anyways.
This is (a) mainly for tests, since a real user probably won't fiddle aimlessly
with the thread count, and (b) will go away once NNC's threading support is
fully baked.
Test Plan: Imported from OSS
Reviewed By: Krovatkin
Differential Revision: D27788199
Pulled By: bertmaher
fbshipit-source-id: 070d04474f15e9689dbdf8cc1fde43050c6506b1
