title: Custom Keras loss function - MSE when y_true and y_pred have different signs
body: <p>I am trying to create a custom Keras loss function where loss = MSE when y_true and y_pred have different signs (e.g. y_true is positive and y_pred is negative or vice versa), else the loss is 0.</p>
  <p>I am not entirely sure how to do this given the loss function has to be vectorised with in-built Keras functions, and I'm not sure how to provide a vector of zeros, given the following loss structure:</p>
  <pre><code>Loss = mean(squared(min(y_true * y_pred, 0)))
  </code></pre>
  <p>Is there a Keras function that can provide a vector of zeros equivalent to the length of y_true?</p>