Clarify and document commit choice for CI jobs (#54967)
Summary:
PRs https://github.com/pytorch/pytorch/issues/53652 and https://github.com/pytorch/pytorch/issues/54693 attempted to increase the consistency of our choice of commit (head vs merge) for CI on PRs, and have so far been unsuccessful. This PR takes a less ambitious approach to the problem by clarifying the choice in one specific way (see the following paragraph) and documenting it in `CONTRIBUTING.md`.
In addition to documentation, this PR also removes the current behavior of our GHA jobs that checkout the PR tip instead of the merge commit. At first glance, this behavior seems to increase consistency (by eliminating the special-case for `ghstack` PRs), but in reality, it actually just means that for non-`ghstack` PRs, the question "Which commit is used in CI?" has *two* answers instead of just one; see the description of https://github.com/pytorch/pytorch/issues/53652 for more details.
Once merged, this PR will unblock other PRs that add modify our GHA workflows in breaking ways, such as https://github.com/pytorch/pytorch/issues/54737.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54967
Test Plan: None.
Reviewed By: walterddr, seemethere
Differential Revision: D27435913
Pulled By: samestep
fbshipit-source-id: 405fb419cf015cf88107d5eb2498cfb5bcb7ce33
