title: Add None dimension in Tensorflow 2 shape
body: <p>I'm trying to change</p>
  <pre><code>Tensorshape(60, 60, 64)
  </code></pre>
  <p>into</p>
  <pre><code>Tensorshape(None, 60, 60, 64)
  </code></pre>
  <p>I'm using it in a model and the dimension None is used for batch I think.</p>
  <p>I can have</p>
  <pre><code>Tensorshape(1, 60, 60, 64)
  </code></pre>
  <p>but it means I have a batch size = 1 (it's works with batch_size = 1 but not with other values)</p>
  <p>How can I do that ?</p>
  <p>Thanks.</p>