Disable TPU tests for app only changes

[hhsecond]

Proposed refactor

Currently TPU tests run for all the PRs. For app only changes, we shouldn't even run those tests. Or rather, enable it only for PL

Motivation

TPUs are limited resources and hence we shouldn't overuse it. Also sometimes takes quite a lot of time to finish the TPU tests which are totally useless for app only changes

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cc @carmocca @akihironitta @Borda

[carmocca]

The TPU job has been moved off the required list, so this can be done now

[rohitgr7]

not just TPUs, let's add all accelerators tests as well if they are running.

[carmocca]

IPUs are marked as required so it cannot be done for them yet. This can only be done for non-required jobs.

[carmocca]

Update: I opened #14539 for some non-TPU jobs.

TPU jobs are trickier since they use CircleCI. I found this guide to follow: https://circleci.com/blog/trigger-circleci-pipeline-github-action/