Fix Matrix-Game-2 VAE decoding

[Arison591]

Summary

• iterate over the latent batch passed to WanVAEWrapper.decode_to_pixel
• restore both regular and cached Matrix-Game-2 VAE decoding

Root cause and impact

decode_to_pixel accepts a tensor named latent, but the decode loop referenced an undefined variable named zs. Both decode paths therefore raised NameError before any generated latent could be converted back to video pixels.

Validation

• python -m ruff check Matrix-Game-2/utils/wan_wrapper.py --select F821,F822,F823
• RTX 5090 CUDA regression with a synthetic VAE covering:
  • regular decoding with a batch of two latents
  • cached decoding with a single latent
  • output shape, CUDA device, float32 conversion, and [-1, 1] clamping
  • the existing cached-decode batch-size assertion