def _split(data, labels, chunk_size, fraction):
    """Return two tuples of couple (data, labels).
    Each tuple containing a multiple of <chunk_size> elements.
    Firts tuple represents an approximative proportion of 1 - <fraction>, while second tuple represents a proportion of <fraction>. 
    """
    n_1 = chunk_size * int((1 - fraction) * data.shape[0] / chunk_size)
    data_1, labels_1 = data[:n_1], labels[:n_1]

    n_2 = chunk_size * int(fraction * data.shape[0] / chunk_size)
    data_2, labels_2 = data[-n_2:], labels[-n_2:]

    return (data_1, labels_1), (data_2, labels_2)