title: Lambda layer in model
body: <p>I'm super confusing with lambda layer during functional API</p>
  
  <p>Some examples show me how to use lambda layer
  and when they stack operation layer, some use lambda and some don't use </p>
  
  <p>For example </p>
  
  <pre><code>D = Input((n,))
  E = Lambda(tf.signal.fft)(D)
  d = Model(~~~~) 
  </code></pre>
  
  <p>or</p>
  
  <pre><code>D = Input((n,))
  E = concatenate(D)
  d = Model(~~~~) 
  </code></pre>