Add split special tokens

[itazap]

What does this PR do?

Fixes #30685 #28648

Before submitting

• [ ] make sure this is saved and used not only as kwargs but also the attribute
• [ ] added tests
• [ ] TODO: add docs

Edit:

• [ ] Moved split_special_token testing to common tests, as it should be supported by both fast and slow models.
• [ ] Verify fast and slow behaviour is identical
• [ ] Tested with T5
• [ ] Note: split_special_tokens is expected in kwargs

[HuggingFaceDocBuilderDev]

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