feat: optimize cfg special cases with guidance schdeule

[stduhpf]

Summary

Avoid running unnecessary diffusion model forward passes when using guidance schedule with scale == 1 (or rather scale == img_cfg)

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[leejet]

The CFG simplification is valid, but skipping these branches also skips work required by other features:

1. Preserve APG behavior
   With APG momentum enabled, removing the unconditional prediction at scale 1 makes AdaptiveProjectedGuidance::forward() return before updating its momentum buffer. Subsequent guided steps therefore produce different results. Removing uncond in three-condition APG also changes the output formula after projection/clipping. Please restrict this optimization to plain CFG until these cases are handled.

2. Keep the ControlNet computation needed by img_uncond
   When text_cfg == img_cfg != 1, skipping the uncond branch also skips computing negative-prompt controls. The remaining img_uncond pass then receives the positive-prompt controls instead. Its required ControlNet computation must remain even when the unconditional diffusion pass is skipped.

3. Invalidate caches for skipped conditions
   With a schedule such as 2 → 1 → 2, the positive cache can be refreshed while the negative cache remains unchanged. When guidance resumes, EasyCache/UCache can reuse the older negative residual based on the newer positive cache state. Please invalidate the skipped condition’s cache or force a refresh when it becomes active again.

[stduhpf]

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