title: custom regression error function for sign accuracy in CNTK
body: <p>I am trying to perform regression task and measure accuracy of correct sign (e.g. predicting stock returns and measuring accuracy of returns in the predicted direction).
  Numpy implementation would look something like this:</p>
  
  <pre><code>def accuracy_error(pred, target):
      return np.dot(np.sign(pred), np.sign(target)) / pred.size
  </code></pre>
  
  <p>How can I implement such error function in CNTK? I do not see cntk.ops.sign. 
  Could you point me to an example?</p>