Implement Shell Model Fetching

[Eve-ning]

Currently, we have train.py setting the model path on the .env file.

I don't think it's a great approach, as it widens the scope of train, which should just train the model, make the model, and possibly return the model path. The problem is that the model path return is not trivial, thus we resorted to I/O-ing to the .env file.

A better solution is to specify a unique model name, which is the pipeline run id, then we can grep the model from opal/models.