Create link prediction GNN model

[JDRomano2]

Users should be able to run the following sample code to build a simple link prediction task on a subgraph containing chemicals, diseases, and genes:

from comptox_ai.ml.nn import NeuralNetwork

nn = NeuralNetwork(model='link-prediction')

# Load the data by calling the new routines you added to comptox_ai.db.graph_db
nn.load_data(node_types=['Chemical', 'Gene', 'Disease'])

# Train the model
nn.fit()

# Return predicted links
nn.predict()