Enhance CI to only build & test affected targets for PRs

[ghpvnist]

🚀 Feature

Leverage tools like bazel-diff to only build/test targets that are affected by the PR during review. However, full CI should still be run when merged to master branch to ensure there are no hidden breakages.

Motivation

Currently, our CI is not great at detecting individual file level changes and running only the affected targets and instead runs full test suite on all PRs. This should save valuable resources that are currently running expensive GPU and TPU tests that are not often necessary during review.

[zhanyong-wan]

Link to bazel-diff: https://github.com/Tinder/bazel-diff

[zhanyong-wan]

I'm not sure we want to run the full checks when merging into master. I think we can leverage bazel-diff there too. This is similar to Google's internal builds, which only build/test affected targets.