title: Add None dimension in tensorflow 2.0
body: <p>I have a tensor <code>xx</code> with shape:</p>
  <pre><code>&gt;&gt;&gt; xx.shape
  TensorShape([32, 32, 256])
  </code></pre>
  <p>How can I add a leading <code>None</code> dimension to get:</p>
  <pre><code>&gt;&gt;&gt; xx.shape
  TensorShape([None, 32, 32, 256])
  </code></pre>
  <p>I have seen many answers here but all are related to TF 1.x</p>
  <p>What is the straight forward way for TF 2.0?</p>