create data iterator with NDArrayIter

def get_mnist_iter(args, kv):
    """
    create data iterator with NDArrayIter
    """
    (train_lbl, train_img) = read_data(
            'train-labels-idx1-ubyte.gz', 'train-images-idx3-ubyte.gz')
    (val_lbl, val_img) = read_data(
            't10k-labels-idx1-ubyte.gz', 't10k-images-idx3-ubyte.gz')
    train = mx.io.NDArrayIter(
        to4d(train_img), train_lbl, args.batch_size, shuffle=True)
    val = mx.io.NDArrayIter(
        to4d(val_img), val_lbl, args.batch_size)
    return (train, val)