docstr: 用DDPG算法更新 actor 和 critic
          
docstr_tok: ["\u7528DDPG\u7b97\u6cd5\u66f4\u65b0","actor","\u548c","critic"]
code: def learn(self, obs, action, reward, next_obs, terminal):
      """用DDPG算法更新 actor 和 critic"""
      critic_loss = self._critic_learn(
          obs, action, reward, next_obs, terminal
      )
      actor_loss = self._actor_learn(obs)
  
      self.sync_target()
      return critic_loss, actor_loss
  
file_path: PARL/examples/tutorials/parl2_dygraph/lesson5/ddpg/algorithm.py
imports: import parl
  import paddle
  import paddle.nn.functional
  from copy import deepcopy
label_desc: Use image node_id 3 for calling the DDPG obj's underlying member method code with example usage: obj.learn(obs, action, reward, next_obs, terminal) and returns: critic_loss,  actor_loss