* PR SPLIT: moving origina changes for adding user defined symbols
* adding gemma test and generalizing gemma converter
* ruff
* update common test
* update serialization test
* deberta v2 tests updates as rust version adds '.' as a user added token, so a space is not added
* removing commented lines
* applying feedback - user only added_tokens to add and check piece.type instead of trainer_spec for user_defined_symbols
* add comment referencing sentencepiece
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