improve saving strategy of sentencepiece tokenizer (#15328)

* add new test

* add a feature to same the sentencepiece tokenizer model when the init file was deleted

* update marian

* update m2m_100

* fix marian

* update speech to text

* override test for layoutxlm

* fix saving bartpho

* remove harcoded values bartpho

* special token string version

* finish bartpho

* override layoutxml test

* add mbart

* move special tokens list

* format

* Revert "format"

This reverts commit 37a40df37903a932c2f951cbd33acb684246bae7.

* simplify list of string of special tokens

* Re-write `self.fairseq_tokens_to_ids ` initialization logic with special tokens
Co-authored-by: Sylvain Gugger <sylvain.gugger@gmail.com>
Co-authored-by: Sylvain Gugger <sylvain.gugger@gmail.com>

tests/test_tokenization_mbart.py

@@ -39,6 +39,7 @@ class MBartTokenizationTest(TokenizerTesterMixin, unittest.TestCase):
     tokenizer_class = MBartTokenizer
     rust_tokenizer_class = MBartTokenizerFast
     test_rust_tokenizer = True
+    test_sentencepiece = True
 
     def setUp(self):
         super().setUp()