Train.

def train(
      self,
      env_fn,
      hparams,
      simulated,
      save_continuously,
      epoch,
      sampling_temp=1.0,
      num_env_steps=None,
      env_step_multiplier=1,
      eval_env_fn=None,
      report_fn=None
  ):
    """Train."""
    raise NotImplementedError()