docstr: update value model self.model with DQN algorithm
          
docstr_tok: ["update","value","model","self.model","with","DQN","algorithm"]
code: def learn(self, obs, action, reward, next_obs, terminal):
      """update value model self.model with DQN algorithm"""
  
      cost = self.cal_bellman_residual(
          obs, action, reward, next_obs, terminal
      )
      optimizer = fluid.optimizer.Adam(
          learning_rate=self.lr, epsilon=1e-3
      )
      optimizer.minimize(cost)
      return cost
  
file_path: PARL/benchmark/fluid/offline-Q-learning/dqn.py
imports: import warnings
  import copy
  import paddle.fluid
  from parl.core.fluid.algorithm import Algorithm
  from parl.core.fluid import layers
label_desc: Use image node_id 3 for calling the DQN obj's underlying member method code with example usage: obj.learn(obs, action, reward, next_obs, terminal) and returns: cost