Make LLVM the default backend for TE (#52314)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/52264
When CPU fusion is enabled without LLVM support in PyTorch, it causes huge slowdown (> 50x). This PR makes the LLVM backend the default backend for TE. Now, an error will be reported if CPU fusion is enabled without LLVM support, to avoid this performance regression.
This PR also updates the tests to not use LLVM, so that the old flow is continued. This is necessary because tests run in CI do not have LLVM.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52314
Reviewed By: ejguan
Differential Revision: D26491294
Pulled By: navahgar
fbshipit-source-id: 74561db1207da805d6d28039450db046ba2988fb
