EulerAncestral add `rescale_betas_zero_snr` (#6187)
* EulerAncestral add `rescale_betas_zero_snr`
Uses same infinite sigma fix from EulerDiscrete. Interestingly the
ancestral version had the opposite problem: too much contrast instead of
too little.
* UT for EulerAncestral `rescale_betas_zero_snr`
* EulerAncestral upcast samples during step()
It helps this scheduler too, particularly when the model is using bf16.
While the noise dtype is still the model's it's automatically upcasted
for the add so all it affects is determinism.
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Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>
