[LoRA] fix: lora unloading when using expanded Flux LoRAs.

[sayakpaul]

What does this PR do?

#10206 added support for properly unloading Control LoRA weights from Flux and allowing the pipeline to be used with from_pipe().

However, this led to the following problem. Say, if we do:

...
pipeline.load_lora_weights("black-forest-labs/FLUX.1-Canny-dev-lora")
pipeline.fuse_lora()
pipeline.unload_lora_weights()
...

This would revert the transformer config back to the original (in_channels=64), rendering this line to output 8 (whereas it should be 16). This is fine for use cases #10206 served but it's not effective for the above use case.

This went uncaught because we didn't have a proper test for it and I didn't run the integration tests for Control LoRA. Sorry about that.

This PR fixes this. To do that, I am adding a flag reset_to_overwritten_params (not fixated on the name) to the unload_lora_weights() of FluxLoraLoaderMixin and suggesting to add a note about it in the docs.

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[a-r-r-o-w]

Maybe a docstring for what reset_to_overwritten_params does and why we added it would be helpful for someone looking at it in future, but no problem if it doesn't make sense to.

The test looks good. Would maybe check if an error is raised when trying to run inference with reset_to_overwritten_params=True. Off to YiYi

[sayakpaul]

Not too fixated on the name. Open to suggestions.

Also open to tackling the problem in better ways.

[sayakpaul]

@a-r-r-o-w docs and docstrings done.

Would maybe check if an error is raised when trying to run inference with reset_to_overwritten_params=True

I am not sure what is meant here.