Add `final_sigma_zero` to UniPCMultistep (#7517)
* Add `final_sigma_zero` to UniPCMultistep
Effectively the same trick as DDIM's `set_alpha_to_one` and
DPM's `final_sigma_type='zero'`.
Currently False by default but maybe this should be True?
* `final_sigma_zero: bool` -> `final_sigmas_type: str`
Should 1:1 match DPM Multistep now.
* Set `final_sigmas_type='sigma_min'` in UniPC UTs
