chebai graph model is undirected

[sfluegel05]

The GNN model in the current config is an undirected one. While there is not a huge difference between undirected and directed in terms of performance, methodologically, the directed version is more sound. See ChEB-AI/python-chebai-graph#7

This is not urgent, but a reminder to train a directed model the next time we do new models for chebifier.

[sfluegel05]

I think I mixed up directed and undirected here. The linked issue (and common sense) tell me that undirected is the correct form (as molecule bonds are not directed either) and directed was the wrong, previous state.

#19 introduced new default models that all use the undirected molecule graph.