[albertvillanova]

This PR stops the Nemotron 3 training templates from training the part of the thinking block that the generation prompt already ends with (<think>\n, or <think></think> with enable_thinking=False).

Fix #8216.
Part of #8109.

Solution

Render the assistant turn first, then emit what the generation prompt pre-fills just before the {% generation %} block when the turn starts with it, and the rest inside. The rendered text is unchanged: only the assistant mask changes.

Changes

• Move the pre-filled part of the thinking block out of the generation block in nemotron_3_nano_training.jinja, nemotron_3_super_training.jinja, nemotron_3_ultra_training.jinja and nemotron_3_5_lightning_training.jinja.
• Remove Nemotron 3 from the strict xfails of test_assistant_masks_exclude_generation_prompt and test_assistant_masks_exclude_generation_prompt_without_reasoning.
• Document the placement in the chat templates README and docs.

---

Note

Medium Risk
Changes which tokens get gradient during Nemotron 3 SFT; incorrect masking would skew training, but scope is limited to training templates and mask boundaries.

Overview
Updates Nemotron 3 training chat templates (Nano, Super, Ultra, 3.5 Lightning) so SFT with assistant_only_loss does not apply loss on tokens the generation prompt already pre-fills at the start of the thinking block (<think>\n, or empty <think></think> when enable_thinking=False).

Each template now builds the assistant turn as a string, prints the matching prefill outside {% generation %}, and wraps only the remainder in {% generation %} — rendered text stays the same; only assistant token masks change. Nemotron 3 entries are removed from the strict xfails on test_assistant_masks_exclude_generation_prompt* in tests/test_chat_template_utils.py, and the chat template docs/README describe this placement.

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[bot-ci-comment]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Pages changed in this PR:

• en/chat_templates

[qgallouedec]

Thanks, checked on my side.