RuntimeError: The size of tensor a (121) must match the size of tensor b (2) at non-singleton dimension 1

Part 1 (2019)
1

I try to recognize if any animal is present (non_blank) or not (blank) on the camera trap images:
image
I tried to change the metric from accuracy to auc_roc_score, but I’m getting the following error:

The size of tensor a (121) must match the size of tensor b (2) at non-singleton dimension 1

My questions are:

  1. As far as I understand, my model is expecting a multiclass problem, but it’s getting binary metric auc_roc_score. What should I change so that I’ll be able to use auc_roc_score?
  2. Do you think that auc_roc_score is not the best one for this purpose? If yes - please share your thoughts.

My folder structure is as follows:
image

I load the data using ImageDataBunch.from_folder:

np.random.seed(42)
size=144
data = ImageDataBunch.from_folder(p, seed=123, padding_mode='zeros', size=size, num_workers=4, resize_method=ResizeMethod.SQUISH,
                                  ds_tfms=get_transforms(max_rotate=2.0, max_zoom=0.9, max_lighting=0.2, max_warp=None)
                                 ).normalize(imagenet_stats)