Move ControlNetXS into Community Folder

[DN6]

What does this PR do?

I don't think we can currently support the implementation of ControlNetXS (#5827) as is in the release version of diffusers.

This PR:

1. Moves it into the community folder so that an implementation is still available to the community.
2. Adds an example inference script that users can use to test the model.

We can revisit this design and adding it into source based on community interest.

Fixes # (issue)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

[sayakpaul]

We need to delete these tests after ControlNetXS has been moved to community.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[sayakpaul]

This works for me. It would make sense to also remove the documentation and add a note about ControlNetXS from the community README. Let's also have Umer as a co-author since they contributed the pipeline initially. WDYT?

[patrickvonplaten]

Can we instead move all the code to the research folder? We should only add code to examples/community that works with from_pretrained(custom_pipeline="...")

[sayakpaul]

@patrickvonplaten done.