Integration example of pygfx with pytorch lightning and pytorch geometric

[royvelich]

I have created an example of how to integrate pygfx with pytorch lightning and pytorch geometric, so one can train a graph neural network on a 3D mesh and render the result as the training progresses. We have discussed this in this discussion. Specifically, the example just trains a GNN to predict the Gaussian curvature at each mesh's vertex, by overfitting the ground-truth curvature.