APPNP

It would be great if you could add [APPNP](https://github.com/klicperajo/ppnp) to your model zoo! It's quite simple and performed best in PyTorch Geometric's benchmark, so people could clearly benefit here as well. :)

The issue requests adding APPNP to the repository's model zoo and links to the ppnp reference implementation. Start by reviewing the existing model-zoo entries and comparing the linked APPNP behavior with the repository's model conventions. Done means APPNP is available in the model zoo and its behavior is covered consistently with the existing models.

Written by the indexing model from the issue text.