docstr: update actor and critic model with DDPG algorithm
          
docstr_tok: ["update","actor","and","critic","model","with","DDPG","algorithm"]
code: def learn(self, obs, action, reward, next_obs, terminal):
      """update actor and critic model with DDPG algorithm"""
      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/parl/algorithms/fluid/ddpg.py
imports: import warnings
  from parl.core.fluid import layers
  from copy import deepcopy
  from paddle import fluid
  from parl.core.fluid.algorithm import Algorithm
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