def METHOD_NAME(checkpoint_dir: str, iteration: int):
    """Save the iteration number of the latest model to be checkpointed.
    Args:
        checkpoint_dir (str): 
            the directory where checkpoint is saved.
        iteration (int): 
            the latest iteration number.
    Returns:
        None
    """
    checkpoint_record = os.path.join(checkpoint_dir, "checkpoint")
    # Update the latest checkpoint index.
    with open(checkpoint_record, "wt") as handle:
        handle.write("model_checkpoint_path: step-{}".format(iteration))