Add capability for consistent GNN training from nekRS

[rickybalin]

This is a feature request to add the ability to train a consistent GNN online from nekRS data.

Consistency is currently enforced by running a pre-processing step (create_halo_info.py) on the nekRS data saved to disk in order to produce the needed data structures. This pre-processing step works in parallel, but does not scale very well.

As a first step:

• Add the pre-processing step to the Trainer class (in trainer.py) in order to generate the necessary data structures for the halo exchange and the consistent loss during online training.

As a second step:

• Look into scaling the Python pre-processing step
• Add the pre-processing step to the gnn plugin and make nekRS compute and provide those data structures along with the other GNN and graph data.