`NBFNet` implementation for homogeneous graphs

[daniel-unyi-42]

Initial version of NBFNet - works for homogeneous graphs.

Possible modifications/extensions of this version:

• support for knowledge graphs (I'm working on this already)
• support for other aggregations, especially PNA - right now, only sum aggregation can be used
• support for other message functions - right now, only distmult can be used
• the users can specify which graph conv layer they want to use, including the incorporation of node features and edge attributes (this would also extend the methodology detailed in the NBFNet paper)
• adding documentation & testing

[FrenkT]

Hi, I am interested in this, so I would be happy to help bring the feature forward. Let me know if it's ok for me to help with writing tests/docs.