title: The meaning of question mark (?) in keras layers input size
body: <p>I'm using python2 with keras, tensorflow.</p>
  
  <pre><code>x = Input((32,), name="input1")
  </code></pre>
  
  <p>I think x's shape is (32,) but print(x) 's result is that 'shape(?,32)'.
  What is means of 'shape(?,32)'?
  And '?' means what and 32 means what..?</p>