Upgrade to PyTorch Geometric 2.x

When we are installing our package, we make sure to check that PyTorch Geometric version is `<2`:
https://github.com/BlueBrain/morphoclass/blob/915309a13857bcad6a913763f2a66ace78183944/install.sh#L58

The `2.0` is out since 2021-09, so we should upgrade our PyTorch Geometric version, after making sure that everything in our code still works fine.

For a list of the new features introduced in PyTorch Geometric 2.x, see [**here**](https://github.com/pyg-team/pytorch_geometric/releases/tag/2.0.0).