src/transformers/convert_slow_tokenizer.py/PegasusConverter/pre_tokenizer
class PegasusConverter:    def pre_tokenizer(self, replacement, add_prefix_space):
        return pre_tokenizers.Sequence(
            [
                pre_tokenizers.WhitespaceSplit(),
                pre_tokenizers.Metaspace(replacement=replacement, add_prefix_space=add_prefix_space),
            ]
        )