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"""
      actor_cost = self._actor_learn(obs)
      critic_cost = self._critic_learn(
          obs, action, reward, next_obs, terminal
      )
      return actor_cost, critic_cost
  
file_path: PARL/examples/tutorials/lesson5/ddpg/algorithm.py
imports: import parl
  from parl import layers
  from copy import deepcopy
  from paddle import fluid
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: actor_cost,  critic_cost