* Add test for SentencePiece not adding special tokens to strings
* Add SentencePieceStringConversionMixin to fix issue 15003
* Fix conversion from tokens to string for most SentencePiece tokenizers
Tokenizers fixed:
- AlbertTokenizer
- BarthezTokenizer
- CamembertTokenizer
- FNetTokenizer
- M2M100Tokenizer
- MBart50Tokenizer
- PegasusTokenizer
- Speech2TextTokenizer
* Fix MarianTokenizer, adjust SentencePiece test to accomodate vocab
* Fix DebertaV2Tokenizer
* Ignore LayoutXLMTokenizer in SentencePiece string conversion test
* Run 'make style' and 'make quality'
* Clean convert_tokens_to_string test
Instead of explicitly ignoring LayoutXLMTokenizer in the test,
override the test in LayoutLMTokenizationTest and do nothing in it.
* Remove commented out code
* Improve robustness of convert_tokens_to_string test
Instead of comparing lengths of re-tokenized text and input_ids,
check that converting all special tokens to string yields a string
with all special tokens.
* Inline and remove SentencePieceStringConversionMixin
The convert_tokens_to_string method is now implemented
in each relevant SentencePiece tokenizer.
* Run 'make style' and 'make quality'
* Revert removal of space in convert_tokens_to_string
* Remove redundant import
* Revert test text to original
* Uncomment the lowercasing of the reverse_text variable
* Mimic Rust tokenizer behavior for tokenizers
- Albert
- Barthez
- Camembert
- MBart50
- T5
* Fix accidentally skipping test in wrong tokenizer
* Add test for equivalent Rust and slow tokenizer behavior
* Override _decode in BigBirdTokenizer to mimic Rust behavior
* Override _decode in FNetTokenizer to mimic Rust behavior
* Override _decode in XLNetTokenizer to mimic Rust behavior
* Remove unused 're' import
* Update DebertaV2Tokenizer to mimic Rust tokenizer
* Deberta tokenizer now behaves like Albert and its `convert_tokens_to_string` is not tested.
* Ignore problematic tests in Deberta V2
* Add comment on why the Deberta V2 tests are skipped
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