Here is the output, I agree it is about the shape but seeing this topic, I’m not so sure : https://forums.fast.ai/t/runtimeerror-the-size-of-tensor-a-115-must-match-the-size-of-tensor-b-64-at-non-singleton-dimension-1/50953/17 10
1/20 * Epoch (train):   0% 0/1 [00:00<?, ?it/s]
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-78-330e12609d6c> in <module>()
     38 
     39     # prints train logs
---> 40     verbose=True,
     41 )

14 frames
/usr/local/lib/python3.6/dist-packages/catalyst/utils/criterion/iou.py in iou(outputs, targets, classes, eps, threshold, activation)
     42         _sum = torch.sum
     43 
---> 44     intersection = _sum(targets * outputs)
     45     union = _sum(targets) + _sum(outputs)
     46     # this looks a bit awkward but `eps * (union == 0)` term

RuntimeError: The size of tensor a (3) must match the size of tensor b (224) at non-singleton dimension 4