ALIGNN model implementation using built-in LineGraph Data Transformation capabilities

[allaffa]

We discussed quite a while about the idea of including the support of the Atomistic Line Graph Neural Network (ALIGNN) model described in the following paper:
https://arxiv.org/abs/2106.01829

There are built-in PyTorch Geometric capabilities that would make that easier to implement. For instance, the construction of the line graph is already supported:
https://pytorch-geometric.readthedocs.io/en/latest/_modules/torch_geometric/transforms/line_graph.html#LineGraph