Add Echo-Memory optional loader and Wan 1.3B example

[WayneJin0918]

Summary

• Add an optional Echo-Memory loader that overlays the released context_k1 DiT fine-tune onto official Wan-AI/Wan2.1-T2V-1.3B.
• Matching keys use original DiffSynth / Wan names, so no conversion is required. Extra action-MLP / SSM / spatial slots are skipped.
• Example + low-VRAM example, plus a short EN/中文 note in the Wan docs.

This is a community fine-tune overlay, not a new backbone. The multi-chunk camera-action / revisit protocol stays in Echo-Memory (arXiv:2606.09803).

Test plan

• python examples/wanvideo/model_inference/Echo-Memory.py with Wan 2.1 1.3B and Hugging Face access to Echo-Team/Echo-Memory
• Confirm the loader prints ~825 overlaid DiT keys and skipped action/SSM extras

Made with Cursor

[Artiprocher]

@WayneJin0918 Thanks for your contribution! We'll merge this PR after some code cleanup.

Also, for future development, feel free to check out our skills guide. It can help you write code that aligns with the framework's best practices more quickly: https://modelscope.cn/collections/DiffSynth-Studio/DiffSynth-Studio-Model-Integration-Skills