Support VLM processors in is_chat_template_prefix_preserving#5558
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Extend
is_chat_template_prefix_preservingto accept VLM processors (not only tokenizers), and update its type hint accordingly.Context: https://github.com/huggingface/trl/pull/5489/changes#r3087655676
Why
For VLMs,
processor.apply_chat_templateis not just an alias forprocessor.tokenizer.apply_chat_template. Checking prefix-preservation on the inner tokenizer can therefore diverge from what actually happens at training time. We want to call the check on the processor whenever one is available.Note
Low Risk
Small, well-scoped change to a utility check and its tests; main risk is introducing an extra PIL/image dependency path when running the prefix-preservation check for processors.
Overview
is_chat_template_prefix_preservingnow accepts either aPreTrainedTokenizeror a VLMProcessorMixinand runs the prefix check viaprocessing_class.apply_chat_template.For processors, the check now builds multimodal messages (including a dummy image) via
prepare_multimodal_messagesso image-token expansion is exercised, and tests add a newrequire_visioncase validating prefix-preservation on a processor template.Reviewed by Cursor Bugbot for commit fee8f7e. Bugbot is set up for automated code reviews on this repo. Configure here.