title: input dimension reshape in Tensorflow conolutional network
body: <p>In the expert mnist tutorial in tensorflow website, it have something like this :</p>
  
  <pre><code>x_image = tf.reshape(x, [-1,28,28,1])
  </code></pre>
  
  <p>I know that the reshape is like </p>
  
  <pre><code>tf.reshape(input,[batch_size,width,height,channel])
  </code></pre>
  
  <p>Q1 : why is the batch_size equals -1? What does the -1 means? </p>
  
  <p>And when I go down the code there's one more thing I can not understand</p>
  
  <pre><code>W_fc1 = weight_variable([7 * 7 * 64, 1024])
  </code></pre>
  
  <p>Q2:What does the image_size * 64 means?</p>