add v prediction (#1386)
* add v prediction
* adat euler for v pred
* velocity -> v_prediction
* simplify
* fix naming
* Update src/diffusers/schedulers/scheduling_euler_discrete.py
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>
* style
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>
