models.py is unused and useless

[hermancollin]

The models.py file contains a class definition for a model that is never used, neither in the training nor anywhere else. Usually, this file is where we put all models for a project, but I only see a child class of ultralytics.YOLO. RetinaNet should be added as well. Such a file can also be useful to abstract the backbone implementation of the model (i.e. it would allow us to call axondetection_model.predict() regardless of if its a YOLO or RetinaNet model).