add flux kontext related files

[tianyuan0211]

What does this PR do?

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 make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[karol-brejna-i]

@tianyuan0211, please update the description: what does the PR change, how to validate it.
Think also, if we want to include this case in examples (README.md).

[regisss]

I agree with @karol-brejna-i + please add this model to README/docs tables (here and there) and add some tests that can be directly taken and adapted from Diffusers: https://github.com/huggingface/diffusers/blob/main/tests/pipelines/flux/test_pipeline_flux_kontext.py