fix(sft): sanitize generation config to prevent save_pretrained crash

[Neonkraft]

Summary

OLMo-3 Think models ship a generation_config.json with temperature/top_p set but do_sample=False. This is harmless at training time (we never call model.generate), but transformers >= 5.x runs strict validation inside GenerationConfig.save_pretrained and rejects the inconsistency:

ValueError: GenerationConfig is invalid:
  - `temperature` is set to 0.6 -- this flag is only used in sample-based
    generation modes. You should set `do_sample=True` or unset `temperature`.

Since every checkpoint save calls model.save_pretrained, this crashes the job at the very first checkpoint. The fix sets do_sample=True in-memory on the trainer's model immediately after construction — local to the run, upstream Hub files unmodified.

AllenAI's open-instruct solves the same issue by stripping the sampling params instead; we prefer setting do_sample=True to preserve the model's recommended inference settings in saved checkpoints.

Type of change

• Bug fix
• New feature
• Refactor
• Performance
• Documentation
• Maintenance

[KonstiNik]

Thanks for the fix. One question:

Is it worth applying this to dpo as well? dpo.py:59-66 constructs DPOTrainer(model=name_or_path) the same way and saves checkpoints via the same path. Suggest moving _sanitize_generation_config to common.py and calling it from both build_sft_trainer and build_dpo_trainer. What do you think?

[Neonkraft]

Yes, makes sense. Fixed.

[KonstiNik]

Great that DPO is covered as well!
As a forward-looking note: HF's strict validator actually checks eight sampling-mode parameters, not just three — min_p, top_h, typical_p, epsilon_cutoff and eta_cutoff aren't covered by the current heuristic (configuration_utils.py:626-654). Fine for OLMo-3 Think specifically, but worth keeping in mind for future models that ship with those params set.

[Neonkraft]

Thanks for pointing this out. Might as well deal with it now, because LeBlanc's Law. Fixed.

[KonstiNik]

Great addition! Approving from my side.