message: Raise error if model to clone is a subclass model.
diff: @@ -91,6 +91,10 @@ def _clone_functional_model(model, input_tensors=None, layer_fn=_clone_layer):
  raise ValueError('Expected `model` argument '
  'to be a functional `Model` instance, '
  'got a `Sequential` instance instead:', model)
  + if not model._is_graph_network:
  + raise ValueError('Expected `model` argument '
  + 'to be a functional `Model` instance, '
  + 'but got a subclass model instead.')
  layer_map = {} # Cache for created layers.
  tensor_map = {} # Map {reference_tensor: corresponding_tensor}
  