Update GraphGPS example

[kasper-piskorski]

Addressing training problems of the GraphGPS example. It is now more aligned with the reference implementation by:

• having a LR scheduler,
• concatenating initial node features and PEs
• having an extra post-GPS MLP layer.
  I plotted loss and test-MAE for the example before and after changes together with results from PNA example (same dataset) for reference.
  
  
  

[rusty1s]

Thank you!