add code for skipping CFG on early steps

[drhead]

Description

• Implements an option to skip CFG on early sampling steps as recommended in https://arxiv.org/abs/2404.07724
• Somewhat awkward to use as it stands. I'll probably change it to floating point for a percentage of sampling steps rather than discrete steps. done. Would recommend anyone that uses it uses it as a quicksetting.
• Probably shouldn't be used for img2img or inpainting (unless using almost 1.0 denoising strength because the assumptions of the paper will almost certainly not hold for those cases (haven't tested this though).
• Improves sample diversity, image quality, and in some cases converges faster. Also often makes higher CFG settings stable. Also, makes the skipped steps twice as fast since the second sample from CFG isn't being made.
• In some cases, may reduce prompt obedience. Particularly, this will also take away your negative prompt on early steps.
• Paper also recommends something similar to Negative Guidance Minimum Sigma, except which skips all steps in that interval instead of every other step. I'll look into adding an option to make NGMS skip all steps towards this end. done, this PR now fully implements the paper. Between the two options, I find I can often get away with skipping CFG on around 1/3 of total sampling steps.

Screenshots/videos:

25 steps, no skipped CFG steps: 25 steps, skipped CFG on first four steps: 50 steps, no CFG skipping: 50 steps, skipped CFG on first eight steps:

As you can see, the image with CFG skipping converged much faster. There are some noticeable side effects on image brightness (prompt on this image contained (dark,:1.3), so this is somewhat less dark than I would typically expect).

Checklist:

• [x] I have read contributing wiki page
• [x] I have performed a self-review of my own code
• [x] My code follows the style guidelines
• [x] My code passes tests

[v0xie]

Awesome PR!

Skip Early Cond should be written in metadata for reproducibility.