[nnc] Enable CPU fusion only when num_threads == 1 (#55621)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55621
Fuser support for thread-level parallelism is a work in progress, so
only fuse when the program is running single-threaded.
ghstack-source-id: 126069259
Test Plan: observe fusion groups formed when torch.get_num_threads==1 vs not
Reviewed By: ZolotukhinM
Differential Revision: D27652485
fbshipit-source-id: 182580cf758d99dd499cc4591eb9d080884aa7ef
