add skip layer guidance to sd 3.5

[vladmandic]

stabilityai recently added support to skip_layer_guidance in their official implementation via this pr:
https://github.com/Stability-AI/sd3.5/pull/11/files

slg adds extra guidance to the original cfg during specific steps.
its calculated as positive output from original model minus positive model output from a model with specific layers removed (e.g. layer 7, 8 and 9 for sd35-medium).

slg is an optional feature, but seems like sai prefers that its enabled for sd35 medium variant.

cc: @yiyixuxu @sayakpaul @DN6 @asomoza

[sayakpaul]

In the works.

[bghira]

well since it's been a week without movement i opened #9880 to cover this base though it doesn't apply for img2img or other things yet. those can be added after.

[vladmandic]

closing as resolved.