title: How to compute AUC for one class in multi-class classification in Keras?
body: <p>I'm training a CNN on a multi-class problem and want to use AUC computed only for one of the classes as a metric. The last layer is a softmax layer with num_classes neurons, labels are sparsely encoded, and data are batched. I believe I need to write a custom function and then pass it as a metric. What would be the best way to do it in Keras? Can I somehow use the built-in function(s)?</p>
  <p>Is there a way to train a model, save it and evaluate this metric for each class afterwards? (through <code>evaluate()</code>?) Or does evaluate only compute metrics specified when compiling?</p>