Integrate GNN watermark module under pygip.protect

[Yushi0618]

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• My code follows the project's coding style
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[Edlison]

Thanks for your work on this!

To keep things consistent with the PyGIP framework, you should place all your implementation files under the pygip/models/defense directory. You can either organize everything into a single .py file or use a dedicated subfolder as a module.

Also, please structure your experiments using a class that inherits from the DefenseBase base class. Each experiment should be encapsulated as a class method for clarity and modularity.

Feel free to continue updating your code, and don’t hesitate to reach out if you have any questions.