fix _optional_components in StableCascadeCombinedPipeline

[yiyixuxu]

currently, StableCascadeCombinedPipeline is missing _optional_components so the combined pipeline is not able to get the correct list of expected modules, i.e. _get_signature_keys (

diffusers/src/diffusers/pipelines/pipeline_utils.py

Line 1555 in c1c4269

def _get_signature_keys(cls, obj):

) will not include the optional components in expected_modules, which will then cause errors in methods using this methods, such as components and to

this fix #7598

[HuggingFaceDocBuilderDev]

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[DN6]

Thanks 👍🏽