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🚨 🚨 🚨 Fix Issue 15003: SentencePiece Tokenizers Not Adding Special Tokens in `convert_tokens_to_string` (#15775)

* 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