[`Sentencepiece`] make sure `legacy` do not require `protobuf` (#25684)

make sure legacy does not require `protobuf`

src/transformers/convert_slow_tokenizer.py

@@ -27,9 +27,10 @@ from tokenizers import AddedToken, Regex, Tokenizer, decoders, normalizers, pre_
 from tokenizers.models import BPE, Unigram, WordPiece
 
 from .utils import is_protobuf_available, requires_backends
+from .utils.import_utils import PROTOBUF_IMPORT_ERROR
 
 
-def import_protobuf():
+def import_protobuf(error_message=""):
     if is_protobuf_available():
         import google.protobuf
 
@@ -38,6 +39,8 @@ def import_protobuf():
         else:
             from transformers.utils import sentencepiece_model_pb2_new as sentencepiece_model_pb2
         return sentencepiece_model_pb2
+    else:
+        raise ImportError(PROTOBUF_IMPORT_ERROR.format(error_message))
 
 
 class SentencePieceExtractor:

src/transformers/models/llama/tokenization_llama.py

@@ -162,11 +162,14 @@ class LlamaTokenizer(PreTrainedTokenizer):
     # Copied from transformers.models.t5.tokenization_t5.T5Tokenizer.get_spm_processor
     def get_spm_processor(self):
         tokenizer = spm.SentencePieceProcessor(**self.sp_model_kwargs)
+        if self.legacy:  # no dependency on protobuf
+            tokenizer.Load(self.vocab_file)
+            return tokenizer
+
         with open(self.vocab_file, "rb") as f:
             sp_model = f.read()
-            model_pb2 = import_protobuf()
+            model_pb2 = import_protobuf(f"The new behaviour of {self.__class__.__name__} (with `self.legacy = False`)")
             model = model_pb2.ModelProto.FromString(sp_model)
-            if not self.legacy:
             normalizer_spec = model_pb2.NormalizerSpec()
             normalizer_spec.add_dummy_prefix = False
             model.normalizer_spec.MergeFrom(normalizer_spec)

src/transformers/models/t5/tokenization_t5.py

@@ -195,11 +195,14 @@ class T5Tokenizer(PreTrainedTokenizer):
 
     def get_spm_processor(self):
         tokenizer = spm.SentencePieceProcessor(**self.sp_model_kwargs)
+        if self.legacy:  # no dependency on protobuf
+            tokenizer.Load(self.vocab_file)
+            return tokenizer
+
         with open(self.vocab_file, "rb") as f:
             sp_model = f.read()
-            model_pb2 = import_protobuf()
+            model_pb2 = import_protobuf(f"The new behaviour of {self.__class__.__name__} (with `self.legacy = False`)")
             model = model_pb2.ModelProto.FromString(sp_model)
-            if not self.legacy:
             normalizer_spec = model_pb2.NormalizerSpec()
             normalizer_spec.add_dummy_prefix = False
             model.normalizer_spec.MergeFrom(normalizer_spec)