title: Max-Pool Gradient Error (Tensorflow 0.9rc0)
body: <p>I am getting an auto-differentiation error for max-pooling. Note that I am feeding in an image, using the gradient of the convnet output wrt the input image, updating the image based on the gradient, and then feeding the image back to the network (think adversarial images). As such, this involves trickier gradient handling and consequently I get this error:</p>
  
  <pre><code>LookupError: No gradient defined for operation 'gradients/MaxPool_grad/MaxPoolGrad' (op type: MaxPoolGrad)
  </code></pre>
  
  <p>Is there an easy work-around?</p>