space of values for MODEL_TYPE, MODEL_WEIGHT not clear to newbs

[jonmay]

some more handholding to teach someone what the space of legitimate values for those variables is helpful. Perhaps a handheld walkthrough that uses an existing huggingface model would be appropriate. On top of that, a walkthrough with a trivially different model that shows e.g. subclassing a huggingface model into a new name, making a small tweak, showing how to add that model to train.py/test.py.