title: get the size of a variable batch dimension
body: <p>assuming the input to the network is a <code>placeholder</code> with variable batch size, i.e.:</p>
  
  <pre><code>x = tf.placeholder(..., shape=[None, ...])
  </code></pre>
  
  <p>is it possible to get the shape of <code>x</code> after it has been fed?  <code>tf.shape(x)[0]</code> still returns <code>None</code>.</p>