FastAI's callbacks for better CNN training -- meet SaveModelCallback.

FastAI has a very flexible callback system that let's you greatly customize your training process. However, some of the pre-built and useful callbacks are not as easy to find without a deep dive into the documentation and to my knowledge, aren't covered in the regular courses. A common question is thus, "how do I automatically save my best model if it happens in the middle of a training run?" and the answer is to use the SaveModelCallback. The purpose of this callback, as the name implies, is to automatically save a new'best loss' model automatically during training. At the end of training, it then conveniently loads the top model so it's ready for you to continue with.