docs: def backward(self, y):
          """
          Transforms a series of triangular matrices y to the packed representation x (numpy)
          
          :param y: unpacked numpy array y, shape self.num_matrices x N x N
          :return: packed numpy array, x, shape self.num_matrices x triangular number
          """
          if self.squeeze:
              y = y[None, :, :]
          ind = np.tril_indices(self.N)
          return np.vstack([y_i[ind] for y_i in y])