src/transformers/convert_slow_tokenizer.py/SpmConverter/__init__
class SpmConverter:    def __init__(self, *args):
        requires_backends(self, "protobuf")

        super().__init__(*args)

        from .utils import sentencepiece_model_pb2 as model_pb2

        m = model_pb2.ModelProto()
        with open(self.original_tokenizer.vocab_file, "rb") as f:
            m.ParseFromString(f.read())
        self.proto = m