title: keras.load_model() can't recognize Tensorflow's activation functions
body: <p>I saved a tf.keras model using <code>tf.keras.save_model</code> functions.
  why <code>tf.keras.load_model</code> throws an exception?</p>
  <p>code example:</p>
  <pre><code>import tensorflow as tf
  from tensorflow import keras
  from tensorflow.keras import layers
  
  model = keras.Sequential([
      layers.Dense(8, activation=tf.nn.leaky_relu),
      layers.Dense(8, activation=tf.nn.leaky_relu)
  ])
  
  tf.keras.models.save_model(
      model,
      'model'
  )
  
  tf.keras.models.load_model('model')
  </code></pre>
  <p>I expect this code to load the model, but it throws an exception:</p>
  <pre><code>ValueError: Unknown activation function:leaky_relu
  </code></pre>