title: How can I create a custom callback in Keras?
body: <p>I'm interested in creating a callback while fitting my keras model. More in detail I'd like to receive a message from a bot telegram with val_acc each time an epoch is over. I know you can add a callback_list as a parameter in <code>classifier.fit()</code> but many callbacks are prebuilt by keras and I don't know how to add a custom one.</p>
  <p>Thank you!</p>