src/transformers/models/marian/convert_marian_to_pytorch.py/save_tokenizer_config
def save_tokenizer_config(dest_dir: Path, separate_vocabs=False):
    dname = dest_dir.name.split("-")
    dct = {"target_lang": dname[-1], "source_lang": "-".join(dname[:-1]), "separate_vocabs": separate_vocabs}
    save_json(dct, dest_dir / "tokenizer_config.json")