Load label image as 1 x height x width integer array of label indices.
        The leading singleton dimension is required by the loss.
        The full 400 labels are translated to the 59 class task labels.

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.
        The full 400 labels are translated to the 59 class task labels.
        """
        label_400 = scipy.io.loadmat('{}/trainval/{}.mat'.format(self.context_dir, idx))['LabelMap']
        label = np.zeros_like(label_400, dtype=np.uint8)
        for idx, l in enumerate(self.labels_59):
            idx_400 = self.labels_400.index(l) + 1
            label[label_400 == idx_400] = idx + 1
        label = label[np.newaxis, ...]
        return label