[data] fix gemma2 eos token (#8480)

Co-authored-by: Yaowei Zheng <hiyouga@buaa.edu.cn>

src/llamafactory/data/template.py

@@ -951,6 +951,22 @@ register_template(
 )
 
 
+# copied from gemma template
+register_template(
+    name="gemma2",
+    format_user=StringFormatter(slots=["<start_of_turn>user\n{{content}}<end_of_turn>\n<start_of_turn>model\n"]),
+    format_assistant=StringFormatter(slots=["{{content}}<end_of_turn>\n"]),
+    format_system=StringFormatter(slots=["{{content}}\n\n"]),
+    format_observation=StringFormatter(
+        slots=["<start_of_turn>tool\n{{content}}<end_of_turn>\n<start_of_turn>model\n"]
+    ),
+    format_prefix=EmptyFormatter(slots=[{"bos_token"}]),
+    stop_words=["<eos>", "<end_of_turn>"],
+    efficient_eos=True,
+    template_class=Llama2Template,
+)
+
+
 # copied from gemma template
 register_template(
     name="gemma3",