code: def lookup_model_by_kind(kind):
      
      model = _known_models.get(kind)
      if model is None:
          raise RuntimeError(f"Model for kind {kind!r} not found.")
      return model
docs: Look up the model instance for a given Datastore kind.
  
      Parameters:
        kind(str)
  
      Raises:
        RuntimeError: If a model for the given kind has not been
          defined.
  
      Returns:
        model: The model class.