feat: add DPM++ (2S) Ancestral implementation for flow models

[wbruna]

This went missing from the version merged on #1374 .

It's mostly based on the calculations from the ComfyUI implementation, but I've replaced the 'sigma_s = 0.9999' hack with an equivalent calculation that avoids that singularity.

For that same sigma==1 case, I've also added an optimization that skips the second model call (rationale on the comments). But that epsilon is arbitrary, and could change the result for sigmas close to 1, so a more conservative approach could be testing for exact 1.0 (although the schedulers may not use a hardcoded 1 constant in that case). The epsilon makes the sigma_s calculation change kind of redundant, but I've kept both because I consider the first just an optimization: everything should keep working if it's changed to sigma==1.0, or disabled.

The opt_first_step dance just avoids the progress bar being silent during the first step.

5 steps (8 NFEs), simple scheduler:

eta 1	eta 0