def backtranslate_data_filenames(self):
    """List of pairs of files with matched back-translated data."""
    # Files must be placed in tmp_dir, each similar size to authentic data.
    return [("cs_mono_en.txt%d" % i, "cs_mono_cs.txt%d" % i) for i in [0, 1, 2]]
List of pairs of files with matched back-translated data.
tensor2tensor/data_generators/translate_encs_cubbitt.py