code: def _critic_learn(self, obs_n, act_n, target_q):
      pred_q = self.Q(obs_n, act_n)
      cost = paddle.mean(F.square_error_cost(pred_q, target_q))
  
      self.critic_optimizer.clear_grad()
      cost.backward()
      self.critic_optimizer.step()
      return cost
  
file_path: PARL/parl/algorithms/paddle/maddpg.py
imports: import parl
  import paddle
  import paddle.nn
  import paddle.nn.functional
  from parl.utils.utils import check_model_method
  from copy import deepcopy
label_desc: Use image node_id 7 for calling the MADDPG obj's underlying member method code with example usage: obj._critic_learn(obs_n, act_n, target_q) and returns: cost