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 (https://github.com/huggingface/diffusers/blob/c1c42698c955959d7ef34af129428f64c6e363bf/src/diffusers/pipelines/pipeline_utils.py#L1555) 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 https://github.com/huggingface/diffusers/issues/7598

[HuggingFaceDocBuilderDev]

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