add checkout PR tip step for quick checks (#49590)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/49590
Reviewed By: samestep
Differential Revision: D25633341
Pulled By: walterddr
fbshipit-source-id: 6e8db1f628f562d7632390bdb7788437cb1bf63d
Author
Rong Rong (AI Infra)
