Add opt-in PiD memory optimizations

[JPPhoto]

Summary

Adds opt-in PiD memory optimizations for GPUs with limited VRAM.

When pid_memory_optimization: true is set in invokeai.yaml, PiD uses float32 sampler intermediates and chunks full-resolution PiT activations. The option applies to every supported PiD decoder. It defaults to false, preserving existing behavior.

Chunking is configured per decode call so cached PiD models cannot retain optimization state between requests.

This PR adds backend tests, generated configuration types, the complete generated settings entry, and PiD/low-VRAM documentation.

Related Issues / Discussions

QA Instructions

1. Add pid_memory_optimization: true to invokeai.yaml.
2. Restart InvokeAI.
3. Run a PiD decode and confirm it completes with reduced peak VRAM.
4. Remove the setting or set it to false, restart, and confirm the original unchunked path is used.

Merge Plan

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)