UniPC Multistep add `rescale_betas_zero_snr` (#7531)
* UniPC Multistep add `rescale_betas_zero_snr`
Same patch as DPM and Euler with the patched final alpha cumprod
BF16 doesn't seem to break down, I think cause UniPC upcasts during some
phases already? We could still force an upcast since it only
loses ≈ 0.005 it/s for me but the difference in output is very small. A
better endeavor might upcasting in step() and removing all the other
upcasts elsewhere?
* UniPC ZSNR UT
* Re-add `rescale_betas_zsnr` doc oops
