title: Log validationSteps in fit_generator
body: <p>In Keras I have the following</p>
  
  <pre><code>MyModel.fit_generator(generator=generatorTraining, epochs=self.nofEpochs,
                                 steps_per_epoch=nofBatchesPerTrainingEpoch, callbacks=callbacks,
                                 validation_data=generatorValidation, validation_steps=nofBatchesPerValidationEpoch)
  </code></pre>
  
  <p>I then add a custom callback to <code>callbacks</code>, to record information about the epoch, training and batches. This I do using the following functions: <code>on_epoch_begin</code>, <code>on_epoch_end</code>, <code>on_train_begin</code>, <code>on_train_end</code>, <code>on_batch_begin</code>, and <code>on_batch_end</code>. I can find other callbacks that I can use in <code>MyModel.evaluate</code>. </p>
  
  <p>But I cannot seem to find a way to get information from the <code>validation_data</code> in a callback, e.g., accuracy. Is this simply not possible or?</p>