def _last_index(self, s):
        self.set_index(max(0, self._find_one_after_last_labeled_index() - 1))
def gradients(self, d_grads, g_grads):
      if self.d_grads is None:
          self.d_grads = [torch.zeros_like(_g) for _g in d_grads]
          self.g_grads = [torch.zeros_like(_g) for _g in g_grads]