title: Plot model with model_to_dot() in Tensorflow Keras not working
body: <p>I am trying to plot my model with the <code>tf.keras.utils.model_to_dot()</code> function but keep getting the following error:</p>
  <pre><code>TypeError: object of type 'Cluster' has no len()
  </code></pre>
  <p>Here is the code I use:</p>
  <pre><code>import tensorflow
  import pydot
  
  model = tf.keras.models.Sequential([
    tf.keras.layers.Flatten(input_shape=(28, 28, 1)),
    tf.keras.layers.Dense(128,activation='relu'),
    tf.keras.layers.Dense(10, activation='softmax')
  ])
  
  model_graph = tf.keras.utils.model_to_dot(model, expand_nested=True, subgraph=True)
  graph = pydot.graph_from_dot_data(model_graph)
  graph.write_png('model.png')
  </code></pre>
  <p>What do I do wrong here?</p>