I encountered this same issue. Thanks to your suggestion of printing input.shape, I figured out the solution. Your input img and target['masks'] need to be the same dimensions. For me, my img was a 3-channel RGB image, but my masks was a 1-channel grayscale image. Because of that, masks was a 2D tensor. By adding a fake channel dimension:
mask = np.expand_dims(mask, axis=0)

I was able to get the code to run.