CI test should use PR commit instead of pulling the latest master

[mrshenli]

IIUC, currently we alway pull the latest master in CI tests, meaning that if we rerun tests on a master commit X, it could pull in more recent commits after X. This would make it harder to find out which commit breaks master.

For example, https://app.circleci.com/jobs/github/pytorch/pytorch/4101228 is a test for #31763, but the its git log contains #31526.

cc @ezyang

[imskr]

I would like to work on this issue! Assign me.

[imskr]

Can I work on this @mrshenli

[huydhn]

Resolve this as this issue is ancient and doesn't apply to the current CI anymore.