Trigger TPU CI only on branches with a pull request

[awaelchli]

What does this PR do?

The current CI for TPU runs on branches created without a pull request. This is a waste of resources. The same rules we have for other workflows should apply to TPU CI.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @carmocca @akihironitta @Borda @kaushikb11

[codecov]

Codecov Report

Merging #10932 (5e2b564) into master (a28b4cd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10932   +/-   ##
=======================================
  Coverage      92%      92%           
=======================================
  Files         177      177           
  Lines       16553    16553           
=======================================
  Hits        15204    15204           
  Misses       1349     1349