code: def token_accuracy(labels, outputs):
    
    weights = tf.to_float(tf.not_equal(labels, 0))
    return tf.metrics.accuracy(labels, outputs, weights=weights)
docs: Compute tokenwise (elementwise) accuracy.
  
    Args:
      labels: ground-truth labels, shape=(batch, seq_length)
      outputs: predicted tokens, shape=(batch, seq_length)
    Returns:
      Two ops, one for getting the current average accuracy and another for
      updating the running average estimate.