E(n) Equivariant GNN Layer and Model

[siavashadpey]

• Implementation of the E(n) Equivariant GNN layer and model, as described by Satorras et al.
• Related to Issue #2298.
• Includes unit test, which tests the equivariance property of the model w.r.t. the coordinates.

I also validated the code by reproducing the results of Experiment 5.2 in the paper: see here. I include this as an example or a long, integration test if interested.

Implemented code as part of Project Option 1 of CS224W (will include a link to the blog post once published)