feat: give feedback about cfg values smaller than 1

[wbruna]

The documentation and examples for many distilled models, such as SDXL Turbo and Z-Image Turbo, specify a CFG scale of 0. So add a few warnings to nudge the user in the right direction.

Edit: removed the cfg cap, as per @stduhpf suggestion.

[stduhpf]

the Diffusers implementation treats any value lesser than 1 the same as 1

Actually, that's not really it. The diffusers implementation uses CFG literally like it was proposed in the original paper (https://arxiv.org/abs/2207.12598v1), with CFG=0 meaning pure conditioned and CFG=-1 meaning unconditioned. It's effectively just an offset of -1 compared to every other implementation.

[wbruna]

That's... unfortunate. So maybe a stronger warning would be better instead.

This was motivated by a Koboldcpp issue (LostRuins/koboldcpp#1880), which mentioned specifically the Z-Image Turbo page. But I believe most GUIs follow the "1 means pure conditioned" convention instead?

[stduhpf]

I'm all for warning the user just in case they set it to such a value by mistake, but in my opinion, overriding the values set by the user is not good behavior.

The user might have set CFG to 0 on purpose to test unconditioned behavior of the model. Or set it to a negative number to see what the negative prompt does.

I say, let the user play with weird parameters values if he wants to, as long as it's not causing crashes.

[leejet]

I think having so many log warnings in the code about the cfg_scale value is a bit annoying. Perhaps it would be sufficient to warn only when cfg_scale < 1.f, since that’s when it may lead to unexpected results.