Add support for APG (adaptive projected guidance)

[stduhpf]

Implements this paper: https://arxiv.org/abs/2410.02416

TLDR:

APG is a set of 3 modilfications for CFG:

• reverse momentum: The CFG update is getting steered away from (or closer to) the previous step's CFG update ( --apg-momentum)
• normalization: the L2 norm of the CFG update is clamped to some value "norm threshold" (--apg-nt)
• projection: the CFG update (out_uncond-out_cond) is orthogonally projected on the same "direction" as out_cond. The final update is linearly interpolated between the original update and the projected update with the parameter "eta" (--apg-eta)

No extra forward pass is required, so the performance cost is negligible.

Thanks mostly to the normalization, but also the projection, this allows to take adventage of very large CFG scales without getting deep-fried output images. I'm not sure how usefull the reverse momentum really is, but it was in the paper so I added it too.

Usage

[your usual command with cfg here] --apg-eta 0 --apg-nt 5 --apg-momentum -0.5

Recommanded values:

• eta between 0 and 1, closer to 0 seems better. In the paper, they recommend setting it to 0 altogether
• norm threshold between 1 and 15
• momentum negative and close to 0