Enable XLA by default on pull requests. (#21991)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21991
ghimport-source-id: 5077e62a613c36256d2b5a2427aa9c3887c4a797
Test Plan: Imported from OSS
Differential Revision: D15907913
Pulled By: ezyang
fbshipit-source-id: c67bb999f02760836d1568c1a3911add3f1538f0
