Add final_sigma_zero to UniPCMultistep

[Beinsezii]

Effectively the same trick as DDIM's set_alpha_to_one and DPM's final_sigma_type='zero' where the extra concatenated sigma is simply zero instead of the prior sigma. Completely fixes the goofy residual noise, particularly with lower step counts.

1st and 2nd order UniPC without the patch

1st and 2nd order UniPC with final_sigma_zero=True

Third order errors out for me even on a clean main branch so that's fun. I'll monkey with it later...

Currently False by default but maybe this should be True like DDIM/DPM?

This could also be represented as the same final_sigma_type='zero'|'sig_min' that DPM uses, but I don't really see why it needs to be a string with only two options. Personally I find this scheme much more intuitive.

No UT right now as I didn't see anything for final_sigma_type='zero' in DPM's UTs.

@yiyixuxu

[yiyixuxu]

ohh thanks for doing this!

could we do final_sigma_type='zero'|'sig_min' like DPM scheduler? we always prefer the string type of argument because it is more flexible. e.g. right now we only have two options, but it will allow us to add more options in the future without increase the number of parameters

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[Beinsezii]

I updated it to use the same values as DPM Multistep, defaulting to "zero" and raising val err on unknown type.

What other possible types might there be? I thought for a bit and nothing really made sense besides sigmas[-1] and zero. I guess you could try to follow the curve by interpolating sigmas [-2:] to create a new non-zero, but that'd still have some residual noise I'd think.

Also if it's Optional[str] should we handle None? Currently I don't as that's how the code is in DPM Multistep.

[yiyixuxu]

looks like some tests are failing because we changed the default
let's add final_sigmas_type = sigma_min to the config for the test?

diffusers/tests/schedulers/test_scheduler_unipc.py

Line 20 in 77103d7

config = {

[yiyixuxu]

What other possible types might there be?

it is hard to predict what's possible in the future, so as a general rule we are trying to always use string type of argument whenever possible

Also if it's Optional[str] should we handle None

we don't have to handle None :)

[Beinsezii]

pytest tests/schedulers passes locally now

[yiyixuxu]

thank you!