class FastAiRobertaTokenizer(BaseTokenizer):
    def __init__(self, tokenizer: RobertaTokenizer, max_seq_len: int=128, **kwargs):
        self._pretrained_tokenizer = tokenizer
        self.max_seq_len = max_seq_len
    def __call__(self, *args, **kwargs):
        return self
    def tokenizer(self, t:str) -> List[str]:
        return ["<s>"] + self._pretrained_tokenizer.tokenize(t)[:self.max_seq_len - 2] + ["</s>"]