fix: respect later-turn single-image placeholders

[primorLee]

Summary

• Resolve the single-image placeholder in the first conversation turn that contains <image>, rather than assuming it is always in turn zero.
• Preserve the existing front-of-conversation fallback when a single-image sample does not include a placeholder.

Testing

• python3 -m unittest discover -s finetune -p 'test_dataset.py' -v
• python3 -m py_compile finetune/dataset.py finetune/test_dataset.py

The focused regression test stubs optional training dependencies and verifies both later-turn placement and the no-placeholder fallback.

[airlsyn]

Thanks @primorLee ! Great catch on the later-turn single-image placeholder handling.

Finetune is no longer maintained in MiniCPM-V — could re-submit to MiniCPM-V-Cookbook , if the corresponding code still exists there.

Happy to review it there.
Appreciate the contribution!

[primorLee]

Thanks for the guidance. I migrated the focused fix and regression coverage to OpenSQZ/MiniCPM-V-CookBook#138, where the corresponding finetune/official/dataset.py code is maintained. Closing this PR to keep the old repository review queue clean.