title: Tensorflow Keras sigmoid activation in functional API
body: <p>I am trying to perform sigmoid activation using the functional API, such as:</p>
  
  <pre><code>next_layer = tf.keras.activations.sigmoid(previous_layer)
  </code></pre>
  
  <p>I am getting the following error:</p>
  
  <pre><code>ValueError: Output tensors to a Model must be the output of a 
  TensorFlow `Layer` (thus holding past layer metadata). Found: 
  Tensor("Relu:0", shape=(?, 200), dtype=float32, device=/device:GPU:0)
  </code></pre>
  
  <p>I can, however, easily call e.g.,</p>
  
  <pre><code>next_layer = tf.keras.layers.LeakyReLU()(previous_layer)
  </code></pre>
  
  <p>Thanks!</p>