docstr: 使用policy model预测输出的动作概率
          
code: def predict(self, obs):
      """使用policy model预测输出的动作概率"""
      prob = self.model(obs)
      return prob
  
file_path: PARL/examples/tutorials/parl2_dygraph/lesson4/policy_gradient/algorithm.py
imports: import parl
  import paddle
  from paddle.distribution import Categorical
  import paddle.nn.functional
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: prob