[CI] Auto-label PRs for better insight and visibility

The Diffusers team is small, which makes the time-consuming process of triaging and reviewing PRs challenging. As a result, smaller well-written PRs can be accidentally ignored when they are easy to merge while larger poorly written PRs can consume precious time when they are hard to quickly understand.

It may be helpful to add some labelling rules to this repository to make these processes easier by surfacing information that can be useful for at-a-glance reviews. This might include labels for:

• Modules and pipelines (llama.cpp example)
• PR size (Github Action)
• Testing related labels such as test-missing (Github Action)

This would allow for better project management by visually highlighting PRs that might be easy to merge as well as PRs that might need more work, without requiring maintainers to immediately open and begin reviewing the contents of the specific requests.

No repository file or test is named. Start by inspecting the repository's existing GitHub Actions and label configuration, then compare the linked llama.cpp, PR size labeler, and testing-rule examples. Done means an agreed set of rules automatically applies useful module, size, or testing labels to pull requests without requiring manual triage.