Add Windows Installation Guide / Requirements

[Dadiya-Harsh]

While setting up Wan2.1 on Windows 10/11, I encountered several installation issues:

• PyTorch → CPU-only build installs by default, CUDA not detected.
• flash_attn → fails to build on Windows.
• No clear guide for CUDA/PyTorch installation on Windows.

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Steps to Reproduce

1. Clone the repo on Windows.

2. Run:

   pip install -r requirements.txt

3. Installation fails (see screenshot below).

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Expected Behavior

• Clear installation instructions for Windows users.

• A dedicated requirements-win.txt with correct dependencies.

• Correct PyTorch + CUDA install command, e.g.:

  pip3 install torch torchvision --index-url https://download.pytorch.org/whl/cu128

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Proposed Solution

• Add a Windows Installation Guide section to the README.
• Provide a requirements-win.txt with Windows-compatible dependencies.
• Mention known incompatibilities (e.g., flash_attn not supported on Windows).
• I have created a solution and pushed it to a new branch: update/installation_guide.

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System Info

• OS: Windows 11 Pro 64-bit
• GPU: NVIDIA RTX (CUDA 12.8 supported)
• Python: 3.10 / 3.11

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Additional Context

Windows users currently need to manually adjust dependencies. Adding official support will improve adoption.

Screenshot of installation error:

[Dadiya-Harsh]

Here this pr solves this issue.

#527

Please review my update and consider merging this PR. It adds a dedicated requirements-win.txt and Windows installation instructions, making setup on Windows 10/11 much easier for users.