[Schedulers] Add methods for conversion between noise and denoised predictions

Guidance methods like APG (#9585 #9626) require conversion between noise and denoised predictions. The computation of these predictions varies across schedulers and depends on `prediction_type`. Currently, this is embedded in `step` functions of the schedulers which complicates implementation of APG.

Adding separate methods for conversion between noise and denoised predictions would simplify implementation and allow easier experimentation.