Add generation markers to the chat template

Same as https://huggingface.co/allenai/AstaBrief_8B/discussions/3, applied to this checkpoint's template (stock Qwen3).

This PR adds `{% generation %}` / `{% endgeneration %}` markers around the assistant output in `chat_template.jinja`, so `return_assistant_tokens_mask=True` returns the assistant tokens, as needed for assistant-only loss (e.g. TRL SFT with `assistant_only_loss=True`). Rendering is unchanged. The mask covers what the model generates, up to and including `<|im_end|>`.

chat_template.jinja

@@ -40,14 +40,16 @@
                 {%- set content = content.split('</think>')[-1].lstrip('\n') %}
             {%- endif %}
         {%- endif %}
+        {{- '<|im_start|>' + message.role + '\n' }}
+        {%- generation %}
         {%- if loop.index0 > ns.last_query_index %}
             {%- if loop.last or (not loop.last and reasoning_content) %}
-                {{- '<|im_start|>' + message.role + '\n<think>\n' + reasoning_content.strip('\n') + '\n</think>\n\n' + content.lstrip('\n') }}
+                {{- '<think>\n' + reasoning_content.strip('\n') + '\n</think>\n\n' + content.lstrip('\n') }}
             {%- else %}
-                {{- '<|im_start|>' + message.role + '\n' + content }}
+                {{- content }}
             {%- endif %}
         {%- else %}
-            {{- '<|im_start|>' + message.role + '\n' + content }}
+            {{- content }}
         {%- endif %}
         {%- if message.tool_calls %}
             {%- for tool_call in message.tool_calls %}
@@ -69,6 +71,7 @@
             {%- endfor %}
         {%- endif %}
         {{- '<|im_end|>\n' }}
+        {%- endgeneration %}
     {%- elif message.role == "tool" %}
         {%- if loop.first or (messages[loop.index0 - 1].role != "tool") %}
             {{- '<|im_start|>user' }}