class_docstr: Hook function that will be called in training.
  
  This class is for clarifying the interface required for Hook functions.
  You don't need to inherit this class to define your own hooks. Any callable
  that accepts (env, agent, step) as arguments can be used as a hook.
docstr: Call the hook.
  
  Args:
      env: Environment.
      agent: Agent.
      step: Current timestep.
code: def __call__(self, env, agent, step):
      """Call the hook.
  
      Args:
          env: Environment.
          agent: Agent.
          step: Current timestep.
      """
      raise NotImplementedError
  
file_path: chainerrl/chainerrl/experiments/hooks.py
imports: from abc import ABCMeta
  from abc import abstractmethod
  import numpy
label_desc: Use image node_id 1 for calling the StepHook obj's underlying member method code with example usage: obj.__call__(env, agent, step) without return types