src/transformers/models/marian/convert_marian_to_pytorch.py/add_special_tokens_to_vocab
def add_special_tokens_to_vocab(model_dir: Path) -> None:
    vocab = load_yaml(find_vocab_file(model_dir))
    vocab = {k: int(v) for k, v in vocab.items()}
    num_added = add_to_vocab_(vocab, ["<pad>"])
    print(f"added {num_added} tokens to vocab")
    save_json(vocab, model_dir / "vocab.json")
    save_tokenizer_config(model_dir)