message: update documentation
  Calling the parent constructor is needed, otherwise it will fail with the following error:
  RuntimeError: It looks like you are subclassing `Model` and you forgot to call `super(YourClass, self).__init__()`. Always start with this line.
diff: @@ -74,6 +74,7 @@ class Model(Network):
  class MyModel(tf.keras.Model):
  def __init__(self):
  + super(MyModel, self).__init__()
  self.dense1 = tf.keras.layers.Dense(4, activation=tf.nn.relu)
  self.dense2 = tf.keras.layers.Dense(5, activation=tf.nn.softmax)
  @@ -94,6 +95,7 @@ class Model(Network):
  class MyModel(tf.keras.Model):
  def __init__(self):
  + super(MyModel, self).__init__()
  self.dense1 = tf.keras.layers.Dense(4, activation=tf.nn.relu)
  self.dense2 = tf.keras.layers.Dense(5, activation=tf.nn.softmax)
  self.dropout = tf.keras.layers.Dropout(0.5)
  