Load label image as 1 x height x width integer array of label indices.
        The leading singleton dimension is required by the loss.

def load_label(self, idx):
        """
        Load label image as 1 x height x width integer array of label indices.
        The leading singleton dimension is required by the loss.
        """
        label = Image.open('{}/SegmentationClass/{}.png'.format(self.dir, idx))
        label = np.array(label, dtype=np.uint8)
        label = label[np.newaxis, ...]
        return label