You will need to make some modifications to the model’s head (which is a classification head in cnn_learner) so that you can train a regression model. Here is a thread that discusses image regression in fastai: Image Regression using fastai
That’s a good point. You could also train it as a regression problem and do any binning afterwards. If the predicted values of the regression model are not good enough by themselves, you can still split them into bins after training. Like this you can also adjust the bin size/width to optimize your metric.