title: How to create a keras layer with a custom gradient in TF2.0?
body: <p>since in TensorFlow 2.0 they plan on unifying all high-level APIs under keras (which I'm not much familiar with) and removing Sessions altogether, I was wondering:</p>
  
  <p><strong>How can I create a custom keras layer that has a custom gradient?</strong></p>
  
  <p>I've seen the (quite limited) <a href="https://keras.io/layers/writing-your-own-keras-layers/" rel="noreferrer">guide</a> on creating custom layers in keras but it doesn't describe what we should do if we want our operation to have a custom gradient.</p>