Add single file support for Stable Cascade

[DN6]

What does this PR do?

Adds single file support to the StableCascadeUNet, which allows users to load in checkpoints published in the original format.

The single file loading logic and mappings are defined within the UNet without a Mixin, since the mapping functions have be accessible to the from_single_file method somehow. It relies on fetching the configs from a hosted repo of configs hosted on the diffusers org.

This is a more practical approach, since a single file pipeline checkpoint would be quite large (~34GB) to load. Single File loading with combined pipelines is also not something we support at the moment and it is particularly challenging with Cascade due to the dtype restrictions in the Prior (Stage C) pipeline, which cannot work with float16, while the decoder (Stage B) can.

Fixes # (issue)

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[DN6]

My biggest concern is that with the current design to support single-file checkpoint loading, we are breaking the design of how we support it for ControlNet and VAE massively. Personally, I like this current approach, though!

I think what we can do is move towards a single SingleFileModelMixin and use class attributes to pass in the functions to convert the checkpoint from original format to diffusers. I can do that in a follow up for all the model classes that use from_single_file

[sayakpaul]

I think what we can do is move towards a single SingleFileModelMixin and use class attributes to pass in the functions to convert the checkpoint from original format to diffusers.

I don't fully understand. Could you maybe elaborate this with pseudocode?

[vladmandic]

great stuff. minor nitpick, there are several references to ControlNetModel, i guess since this implementation is copied from there?

[sayakpaul]

The docstring is. https://github.com/huggingface/diffusers/pull/7295 should clean it up.