old_contents: def batch_states(states, xp, phi):
      """The default method for making batch of observations.
  
      Args:
          states (list): list of observations from an environment.
          xp (module): numpy or cupy
          phi (callable): Feature extractor applied to observations
  
      Return:
          the object which will be inputed to the model.
      """
  
      states = [phi(s) for s in states]
      return xp.asarray(states)
  
new_contents: def batch_states(states, xp, phi):
      """The default method for making batch of observations.
  
      Args:
          states (list): list of observations from an environment.
          xp (module): numpy or cupy
          phi (callable): Feature extractor applied to observations
  
      Return:
          the object which will be given as input to the model.
      """
  
      states = [phi(s) for s in states]
      return xp.asarray(states)
  
subject: fix docstring