Implement GNN for MP-HP link prediction task in Embiggen

[justaddcoffee]

@GuoJing implemented a GNN for MP-HP link prediction in DGL, and it seemed to work well. We can implement this in Embiggen without too much effort, which should make it easier to run this task automatically in NEAT.

Here is an example GNN in the develop branch of Embiggen.

Here is a helper function to make a link prediction GNN