Fixes#4150.
Pre-PR, `_fix_chat_template` only patched templates where a trailing `{{ ... }}` expression followed the last `{% endfor %}`. ChatML templates (Hermes, Magnum, Phi-4, etc.) that end cleanly at `{% endfor %}` with no generation-prompt block were left unchanged, so the outer `fix_chat_template` raised:
```
RuntimeError: Unsloth: The tokenizer `...` does not have a
{% if add_generation_prompt %} for generation purposes.
```
This commonly shows up when a downstream tool (LlamaFactory, Axolotl) re-serializes the tokenizer during LoRA save and strips the generation-prompt block.
This PR adds a second branch to `_fix_chat_template` that fires when:
- the content after the last `{% endfor %}` is empty modulo Jinja `{# ... #}` comments,
- the scrubbed template contains `<|im_start|>` and `<|im_end|>`,
- and the scrubbed template does not already mention `add_generation_prompt`.
The assistant-turn separator is inferred from the template itself (preferring an explicit `'<|im_start|>assistant<sep>'` literal, then the unique `message['role'] + '<sep>'` from role concatenations, then `<|im_sep|>` for Phi-4-mini mixed-separator templates, then `\n`), so Phi-4-style templates are not silently corrupted with the wrong separator.
Verified against the existing chat-template corpus:
- Hermes-3, Magnum-v2, Phi-4-mini, Phi-4 multi-sep, ChatML with trailing whitespace, ChatML with trailing Jinja comment, dot-access `message.role`, split-literal `'<|im_start|>assistant'`: all repaired with the correct assistant prefix.
- Already-fixed ChatML templates: idempotent NOP.
- Trap templates with `<|im_start|>` only inside a Jinja comment: correctly not rewritten.
- Llama-3, Gemma-3, Qwen2.5 (non-ChatML): byte-identical.
- Mistral family (5 models including Mistral-Nemo, Mistral-Small-24B, Mixtral): byte-identical, protected both by the structural guard (no ChatML tokens) and the existing name-based exemption in `load_correct_tokenizer`.
- Qwen family (14 models including Qwen2.5, Qwen3, Qwen3-Coder, QwQ, VL, Math, Qwen3-Guard): byte-identical.
End-to-end reproduction: Hermes-3 LoRA SFT, save with stripped chat_template, reload. Pre-PR code path raises the RuntimeError above. Post-PR reload loads cleanly, patches the template at load time, and `apply_chat_template(add_generation_prompt=True)` produces the correct `<|im_start|>assistant\n` prefix.
14ab6fbfae