Cleanup mypy lint job (#54260)
Summary:
Update to checkout v2
Delete "Get HEAD commit SHA" step
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54260
Reviewed By: samestep
Differential Revision: D27160678
Pulled By: malfet
fbshipit-source-id: d1afe4f1cf0046cfb93de583ee123b4db5b25f9a
