Fix HiDiffusion ratio overrides and SDXL T2 behavior

[DustyShoe]

Summary

This is a follow-up to #8787 that fixes correctness and compatibility issues found in the existing HiDiffusion integration.

• Restore the original staged SDXL RAU-Net schedule semantics and apply ratio thresholds against logical denoising steps instead of individual UNet forwards.
• Reuse window-attention shifts within a scheduler step and prevent schedule drift in guidance paths that perform multiple UNet calls.
• Fix feature-map compatibility for Regional Guidance, ControlNet, and T2I-Adapter while RAU-Net is active.
• Keep Automatic Ratios separate from manual T1/T2 slider values, preserve both modes through metadata recall and persisted-state migration, and validate T2 <= T1.
• Stop applying HiDiffusion to the separate SDXL refiner stage, which is not covered by the upstream implementation.
• Correct the T1/T2 UI descriptions, feature documentation, and vendored-source attribution.

Related Issues / Discussions

Follow-up to #8787.

Upstream reference: https://github.com/megvii-research/HiDiffusion

QA Instructions

1. Generate SDXL at 2048x2048 with 50 DDIM steps and Automatic Ratios; inspect the transitions around the early RAU-Net boundary and T1.
2. Disable Automatic Ratios and confirm the previous manual T1/T2 values are preserved. Compare with manual T1=0.4, T2=0.16 using the same seed.
3. Recall images generated in automatic and manual modes and confirm the correct mode and values are restored.
4. Repeat representative txt2img cases with Euler A.
5. Smoke-test img2img, inpainting, Regional Guidance, ControlNet, and T2I-Adapter.

Merge Plan

No special merge handling is required.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)