Add example directory and example scripts of using a pretrained model

Created by: AMHermansen

With the introduction of #567 we now have pretrained models in the library. But we don't have any examples showing how they might be used for inference. I think creating a new directory examples/06_pretrained and adding some example scripts for various use cases of using pretrained models.

Some use-cases that come to mind

1. Predicting on a dataset with/without config files.
2. Benchmarking multiple models against one another on the same dataset.
3. Downstream training. (Freezing the gnn-part, but replacing the task/final MLP (this might not be possible in the current GraphNeT and might deserve it's own issue)).