Add some logic to pass the correct github ref to mcp script

[bandish-shah]

What does this PR do?

Adds logic to the pytest-gpu workflow to pass the correct Github reference to the mcp test runner. If the workflow is triggered by an event in a pull request, it will use the PR number. Otherwise the commit SHA will be used if the run is on a protected branch, else the branch name is used.

Testing

Temporarily added pull_request trigger to kick off the daily workflow in this PR:
https://github.com/mosaicml/composer/actions/runs/4247384552/jobs/7385370010

GPU runs failed due to a failing test in dev so this is expected. Fix for that will be in another PR but should not block this PR.

Before submitting

• Have you read the contributor guidelines?
• Did you run pre-commit on your change? (see the pre-commit section of prerequisites)

To Do

• Remove pull_request trigger that was added to test daily workflow

[mvpatel2000]

Approving to unblock -- I assume this will take a few iterations.... Assuming ur testing as much as possible before merging

[bandish-shah]

I tested the daily by manually adding the pull_request trigger in the PR. Just removed before merging.