Add opt-in low-VRAM mode for Wan generation

[JPPhoto]

Summary

Adds an opt-in Wan memory optimization mode:

wan_memory_optimization: true

The option defaults to false.

When enabled, Invoke:

• Limits resident Wan transformer weights to about 2 GiB and streams remaining layers from RAM.
• Enforces the residency limit after model-cache loading, including already-resident experts and explicit VRAM cache configurations.
• Chunks pointwise transformer operations while preserving global self-attention and cross-attention.
• Compacts TI2V per-token timestep conditioning to its unique timestep states.
• Streams untiled causal VAE decode chunks directly into the MP4 writer instead of retaining the complete RGB video in GPU or system memory.
• Uses a streaming-aware VAE working-memory estimate.

This applies to Wan image and video denoise, including dual-expert A14B models.

The main tradeoff is speed: aggressive weight streaming can make generation substantially slower and requires enough system RAM for offloaded weights. Spatially tiled VAE decode retains its existing path.

Related Issues / Discussions

Related design reference: #9460.

QA Instructions

1. Add this setting to invokeai.yaml:

   wan_memory_optimization: true

2. Restart Invoke.

3. Run Wan image and video generation with representative configurations:

   • TI2V-5B
   • A14B T2V or I2V
   • A14B dual-expert generation
   • CFG enabled and disabled

4. Confirm the log contains:

   Wan memory optimization: limiting resident transformer weights to about 2 GiB
   

5. Confirm the model-cache log reports substantially reduced resident transformer weights and peak VRAM is lower.

6. Confirm generated videos have the expected dimensions, frame count, duration, and playback.

7. Repeat with wan_memory_optimization: false and confirm existing behavior is unchanged.

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)