title: keras run validation after every N training epochs
body: <p>I am using the following function to train/validate my model:</p>
  
  <pre><code>model.fit_generator(
      train_generator,
      steps_per_epoch=nb_train_samples // batch_size,
      epochs=epochs,
      validation_data=validation_generator,
      validation_steps=nb_validation_samples // batch_size,
      verbose=2, workers=12)
  </code></pre>
  
  <p>The above function runs validation every epoch. My validation data is quite large so I want to run it every N epochs instead. How can I do that?</p>