def prepare_model(self, sess, allow_initialize=True):
    """Initialize the model and if necessary launch the queue runners."""
    if self._follower:
      self.wait_for_initialization()
    else:
      self._init_model(sess, allow_initialize)

    if sess is not self._sess:
      if self.threads:
        raise ValueError('You must call stop_queues() before '
                         'starting a new session with QueueRunners.')
      self._sess = sess
    self._start_threads(sess)
Initialize the model and if necessary launch the queue runners.