Chat template: update for processor

[zucchini-nlp]

What does this PR do?

CI will be red until #35558 is merged, we need batched support for all VLMs. But feel free to take a first review, anyone who wants :)

This PR adds:

• Support for batched inputs, which I forgot to add the last time
• Tiny hack for add_special_tokens as per @Rocketknight1's request
• More tests for chat templates in the Mixin, so we don't write the same test per model. Note that for test to run, we need to have the correct chat_template in processor, not a dummy one

[HuggingFaceDocBuilderDev]

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