[MPNet] Add slow to fast tokenizer converter (#9233)

* add converter

* delet unnecessary comments

tests/test_tokenization_mpnet.py

@@ -17,6 +17,7 @@
 import os
 import unittest
 
+from transformers import MPNetTokenizerFast
 from transformers.models.mpnet.tokenization_mpnet import VOCAB_FILES_NAMES, MPNetTokenizer
 from transformers.testing_utils import require_tokenizers, slow
 
@@ -27,7 +28,9 @@ from .test_tokenization_common import TokenizerTesterMixin
 class MPNetTokenizerTest(TokenizerTesterMixin, unittest.TestCase):
 
     tokenizer_class = MPNetTokenizer
-    test_rust_tokenizer = False
+    rust_tokenizer_class = MPNetTokenizerFast
+    test_rust_tokenizer = True
+    space_between_special_tokens = True
 
     def setUp(self):
         super().setUp()