title: How to define a layer with multiple input tensors(tensors with different shape)
body: <p>I want to define a layer that can have multiple inputs. I found an example on keras doc here
  <a href="https://keras.io/layers/writing-your-own-keras-layers/" rel="nofollow noreferrer">https://keras.io/layers/writing-your-own-keras-layers/</a>
  But i don't understand the call function. 
  e.g. if i want to give <em>a</em> an assignment  like input layer 1, b is assigned with input layer  2. How to define the call? </p>