title: TF2 / Keras slice tensor using [:, :, 0]
body: <p>In TF 2.0 Beta I'm trying:</p>
  
  <pre><code>x = tf.keras.layers.Input(shape=(240, 2), dtype=tf.float32)
  print(x.shape) # (None, 240, 2)
  a = x[:, :, 0]
  print(a.shape) # &lt;unknown&gt;
  </code></pre>
  
  <p>In TF 1.x I could do:</p>
  
  <pre><code>x = tf1.placeholder(tf1.float32, (None, 240, 2)
  a = x[:, :, 0]
  </code></pre>
  
  <p>and it would work fine. How do I achieve this in TF 2.0? I think </p>
  
  <pre><code>tf.split(x, 2, axis=2)
  </code></pre>
  
  <p>may work, however I'd like to use slicing rather than hard coding the 2 (dim of axis 2).</p>