[Sentencepiece] make sure legacy do not require protobuf

[ArthurZucker]

What does this PR do?

Fixes the get_spm_processor() function to make sure that if protobuf is not installed, we can still initialize the model when legacy=False.
Just realized that make fixup fails without protobuf 😓
fixes #25753

>>> import tensorflow as tf
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
Cell In[2], line 1
----> 1 import tensorflow as tf

File /opt/conda/envs/py39/lib/python3.9/site-packages/tensorflow/__init__.py:37
     34 import sys as _sys
     35 import typing as _typing
---> 37 from tensorflow.python.tools import module_util as _module_util
     38 from tensorflow.python.util.lazy_loader import LazyLoader as _LazyLoader
     40 # Make sure code inside the TensorFlow codebase can use tf2.enabled() at import.

File /opt/conda/envs/py39/lib/python3.9/site-packages/tensorflow/python/__init__.py:37
     29 # We aim to keep this file minimal and ideally remove completely.
     30 # If you are adding a new file with @tf_export decorators,
     31 # import it in modules_with_exports.py instead.
     32 
     33 # go/tf-wildcard-import
     34 # pylint: disable=wildcard-import,g-bad-import-order,g-import-not-at-top
     36 from tensorflow.python import pywrap_tensorflow as _pywrap_tensorflow
---> 37 from tensorflow.python.eager import context
     39 # pylint: enable=wildcard-import
     40 
     41 # Bring in subpackages.
     42 from tensorflow.python import data

File /opt/conda/envs/py39/lib/python3.9/site-packages/tensorflow/python/eager/context.py:28
     25 from absl import logging
     26 import numpy as np
---> 28 from tensorflow.core.framework import function_pb2
     29 from tensorflow.core.protobuf import config_pb2
     30 from tensorflow.core.protobuf import coordination_config_pb2

File /opt/conda/envs/py39/lib/python3.9/site-packages/tensorflow/core/framework/function_pb2.py:7
      5 import sys
      6 _b=sys.version_info[0]<3 and (lambda x:x) or (lambda x:x.encode('latin1'))
----> 7 from google.protobuf import descriptor as _descriptor
      8 from google.protobuf import message as _message
      9 from google.protobuf import reflection as _reflection

ModuleNotFoundError: No module named 'google.protobuf'

fixes #25667

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sgugger]

Thanks for the fix!