transformers.RobertaTokenizer
https://huggingface.co/docs/transformers/model_doc/roberta#transformers.RobertaTokenizer
transformers.PreTrainedTokenizer
を継承
https://github.com/huggingface/transformers/blob/v4.18.0/src/transformers/models/roberta./tokenization_roberta.py#L103
from_pretrained
の実装はPretrainedTokenzerの親クラスの
transformers.PreTrainedTokenizerBase