src/transformers/convert_slow_tokenizer.py/BigBirdConverter/post_processor
class BigBirdConverter:    def post_processor(self):
        return processors.TemplateProcessing(
            single="[CLS]:0 $A:0 [SEP]:0",
            pair="[CLS]:0 $A:0 [SEP]:0 $B:1 [SEP]:1",
            special_tokens=[
                ("[CLS]", self.original_tokenizer.convert_tokens_to_ids("[CLS]")),
                ("[SEP]", self.original_tokenizer.convert_tokens_to_ids("[SEP]")),
            ],
        )