Initialize the graph and session, if this has not yet been done.

def _lazily_initialize(self):
    """Initialize the graph and session, if this has not yet been done."""
    # TODO(nickfelt): remove on-demand imports once dep situation is fixed.
    import tensorflow.compat.v1 as tf
    with self._initialization_lock:
      if self._session:
        return
      graph = tf.Graph()
      with graph.as_default():
        self.initialize_graph()
      # Don't reserve GPU because libpng can't run on GPU.
      config = tf.ConfigProto(device_count={'GPU': 0})
      self._session = tf.Session(graph=graph, config=config)