Incorrect Validation Warning When Using Multimodal Context With No Other Prompt References

When an image is passed as context using the multi_modal_context parameter and no other column is referenced in the prompt, the validator throws an incorrect warning, PROMPT_WITHOUT_REFERENCES.

See visual-question-answering.ipynb to reproduce the error.

从 nemo/NeMo-Data-Designer/advanced/multimodal/visual-question-answering-using-vlm.ipynb 开始，运行 visual-question-answering 示例以复现 PROMPT_WITHOUT_REFERENCES 警告。追踪 multi_modal_context 使用的验证器路径；当仅包含图像的上下文不再产生此警告，而真正不含引用的 prompt 仍然会产生此警告时，即表示完成。