docstr: Policy Gradient algorithm
  
  Args:
      model (parl.Model): forward network of the policy.
      lr (float): learning rate of the policy model.
code: def __init__(self, model, lr=None):
      """Policy Gradient algorithm
  
      Args:
          model (parl.Model): forward network of the policy.
          lr (float): learning rate of the policy model.
      """
  
      self.model = model
      assert isinstance(lr, float)
      self.lr = lr
  
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 1 to create a new PolicyGradient object from inherited base classes: Algorithm with example: obj = PolicyGradient(model, lr)