fix modular pipeline class resolution for families outside the auto-pipeline mappings

[akshan-main]

What does this PR do?

ModularPipeline.from_pretrained on a standard repo finds the modular class through the auto-pipeline mappings. LTX isn't in them, so it fell back to the base ModularPipeline, which has no default blocks, and __init__ crashed on an unbound variable:

ModularPipeline.from_pretrained("Lightricks/LTX-Video")
# UnboundLocalError: cannot access local variable 'blocks_class' where it is not associated with a value

Bare ModularPipeline() crashed the same way. This adds LTXPipeline and LTXImageToVideoPipeline to the text2video and image2video auto mappings, so LTX-Video loads as LTXModularPipeline with LTXAutoBlocks, and raises a clear ValueError when no blocks class can be resolved (the old warning branch was unreachable, and the next line dereferences self._blocks anyway).

Found while verifying #14730 on real checkpoints.

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Who can review?

@yiyixuxu

[github-actions]

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[yiyixuxu]

hi @akshan-main
I think you can just add ltx into the mapping, we don't have to support any pipeline that's not listed in the map (it's sort of a legacy use case anyway to support hub repos without a modular_model_index.json)

however, agree that we should have better warn/error message for this case

[akshan-main]

done, added LTXPipeline and LTXImageToVideoPipeline to the text2video/image2video auto mappings and dropped the folder lookup. kept the error.

[yiyixuxu]

thanks

[HuggingFaceDocBuilderDev]

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[yiyixuxu]

actually here it loads from a model_index.json too, can you just add the assert lines here? and drop the other tests?

assert pipe.__class__.__name__ == 
assert pipe.blocks.__class__.__name ==