add tutorials/examples on Equivariant GNNs

[CarloLucibello]

From @rbSparky comment #412 (comment)

After further discussion(on Slack), since it has been decided that it would be more valuable to implement more relevant applications, this discussion thread can now be used for writing the tutorial for "Molecular Property Prediction using EGNNs". It can serve as a part 2 to the current published tutorial, adding a regression task(using QM9) since currently only classification is shown
Paper
Dataset to be used: QM9
The above techniques from DropMessage can be incorporated as data augmentation technique (to show the user how to write custom functions for tasks like these)
This tutorial is in progress!