docstr: use policy model self.model to predict the action probability
          
code: def predict(self, obs):
      """use policy model self.model to predict the action probability"""
      return self.model(obs)
  
file_path: PARL/parl/algorithms/fluid/policy_gradient.py
imports: import warnings
  import paddle.fluid
  from parl.core.fluid.algorithm import Algorithm
  from parl.core.fluid import layers
label_desc: Use image node_id 2 for calling the PolicyGradient obj's underlying member method code with example usage: obj.predict(obs) and returns: self