message: Fix the functional model loading with nested sequential model.
  The nested sequential model is created with _is_graph_network = False, the current instance check is not strong enough.
diff: @@ -1017,7 +1017,9 @@ def _map_subgraph_network(inputs, outputs):
  def _should_skip_first_node(layer):
  """Returns True if the first layer node should not be saved or loaded."""
  # Networks start with a pre-existing node linking their input to output.
  - return isinstance(layer, Functional)
  + # For a sequential model, it is first created with _is_graph_network = False,
  + # we have to keep the _is_graph_network check here.
  + return isinstance(layer, Functional) and layer._is_graph_network
  def _deserialize_keras_tensors(kwargs, layer_map):
  