Skip test_mm on XLA (#35709)
Summary:
https://github.com/pytorch/pytorch/issues/34891 caused a 15 minute regression in XLA test timing when it inadvertently added this test to XLA -- I think it was intended to only add this test to CUDA.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35709
Test Plan: The XLA test job should return from ~75 to ~60 minutes.
Reviewed By: malfet
Differential Revision: D20748176
Pulled By: yns88
fbshipit-source-id: b50227a35bcbf2915b4f2013e2a4705e905d0118
