title: Under what condition can we use @tf.function?
body: <p>I have heard that <code>tf.function</code> can build graph and accelerate computation. But when can we use it?</p>
  <p>From <a href="https://stackoverflow.com/questions/63306632/when-we-should-use-tf-function-decorator">another answer</a>, seems like:</p>
  <blockquote>
  <p>when we are not using built-in functions</p>
  </blockquote>
  <p>Any more specific answer?</p>