[MM] Add support for probing and loading SDXL VAE checkpoint files

[lstein]

Summary

This adds support for probing and converting SDXL VAE checkpoint files. It is a lame solution because the base-determining method simply looks for the pattern "SDXL" in the filename. A quick inspection of SD1 vs SDXL VAEs found no differences in the state dict weights, so these can't be used as the basis of discrimination.

Conversion seems to work using the diffusers library. However, the need to convert will be obsoleted by PR #6510 .

Related Issues / Discussions

See Discord thread at https://discord.com/channels/1020123559063990373/1020123559831539744/1253157322377793557

QA Instructions

I've tested on the Civitai VAE https://civitai.com/models/140686/fix-fp16-errors-sdxl-lower-memory-use-sdxl-vae-fp16-fix-by-madebyollin. The results aren't great - the colors are washed out - but I don't know if this is a problem with the VAE or with the conversion.

Merge Plan

Merge when approved.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Documentation added / updated (if applicable)

[hipsterusername]

Should we just punt on this? Especially since it seems like it's going to be unreliable - I can only think of one VAE named with "SDXL" in it

[lstein]

Should we just punt on this? Especially since it seems like it's going to be unreliable - I can only think of one VAE named with "SDXL" in it

I did a quick search for all SDXL VAEs on Civitae, and each one has the string "XL" in the name. None of the SD 1.5 models contain this string. I changed the regular expression to look for "XL", and probing will work for now. If some future model has its base type incorrectly called the user can always edit it.

I'm going to go ahead and merge in. This is a rare use case anyway.