title: TF Keras ModelCheckpoint filepath batch number
body: <p>I am using <code>ModelCheckpoint</code> to save checkpoints every 500 batches in every epoch. It is documented here <a href="https://www.tensorflow.org/api_docs/python/tf/keras/callbacks/ModelCheckpoint" rel="nofollow noreferrer">https://www.tensorflow.org/api_docs/python/tf/keras/callbacks/ModelCheckpoint</a>.</p>
  
  <p>How would I set <code>filepath</code> to include the batch number? I know I can use <code>{epoch}</code> and parameters in <code>logs</code>. </p>