Modular backend - overrides

[StAlKeR7779]

Summary

Initial implementation of overrides in modular backend, should be used in inpaint and tiled extensions(also in preview extension after preview event rewrite). Created PR now just to have ability to discuss. To be precise - need to decide about override name for unet call and how better to implement arguments in overrides.

Related Issues / Discussions

#6606 https://invokeai.notion.site/Modular-Stable-Diffusion-Backend-Design-Document-e8952daab5d5472faecdc4a72d377b0d

QA Instructions

Run with set USE_MODULAR_DENOISE environment.

Merge Plan

Discuss, then merge.

Checklist

• [x] The PR has a short but descriptive title, suitable for a changelog
• [ ] Tests added / updated (if applicable)
• [ ] Documentation added / updated (if applicable)