Update Stable Cascade Conversion Scripts

[DN6]

What does this PR do?

1. Adds the conversion script for Stable Cascade Lite
2. Updates the conversion script for Stable Cascade to support the variant argument

Fixes # (issue)

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[sayakpaul]

Looks clean!

Do we want to update the docs with a mention of the lite checkpoint as well?

Also is it not possible to repurpose the existing cascade conversion script to account for the lite variant?

[DN6]

Also is it not possible to repurpose the existing cascade conversion script to account for the lite variant?
Based on discussion here: https://huggingface.slack.com/archives/C065E480NN9/p1709892168966619?thread_ts=1709890560.672549&cid=C065E480NN9

[sayakpaul]

I think the doc entry for the lite variant is pending?

[yiyixuxu]

thanks!

[sayakpaul]

@DN6 let's add the docs and ship this!

[sayakpaul]

@DN6 will merge #7274 after the CI is done with. Could you leverage the functionalities from that PR to refactor this PR a bit i.e., move the state dict munging functions to single_file_utils.py, make single-file support for Lite is working, etc.?

[DN6]

Could you leverage the functionalities from that PR to refactor this PR a bit i.e., move the state dict munging functions to single_file_utils.py, make single-file support for Lite is working, etc.?

Done. Also added a few flags to make the scripts for flexible (skipping stages, optionally creating combined pipeline, changing save folder names etc)

[sayakpaul]

This looks solid to me except for the comment on skip_* args.

[sayakpaul]

Feel free to merge once #7271 (comment) is addressed.