Fix to 'Model' object has no attribute '_container_nodes' error when using...

Fix to 'Model' object has no attribute '_container_nodes' error when using tf.keras.utils.plot_model().

Fix to #17633. Duplicate of #17658
'Model' object has no attribute '_container_nodes' error when using tf.keras.utils.plot_model().
Replaced
if node_key in model._container_nodes:
with
if node_key in model._network_nodes: # pylint: disable=protected-access

in tensorflow\python\keras_impl\keras\utils\vis_utils.py.