[model] support gemma3 (#7273)

src/llamafactory/data/template.py

@@ -310,6 +310,8 @@ class Template:
 
 @dataclass
 class Llama2Template(Template):
+    r"""A template that fuse the system message to first user message."""
+
     @override
     def _encode(
         self,
@@ -815,10 +817,29 @@ register_template(
     name="gemma",
     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=["<end_of_turn>"],
+    template_class=Llama2Template,
+)
+
+
+# copied from gemma template
+register_template(
+    name="gemma3",
+    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=["<end_of_turn>"],
+    mm_plugin=get_mm_plugin("gemma3", image_token="<image_soft_token>"),
+    template_class=Llama2Template,
 )
 
 
@@ -1255,6 +1276,7 @@ register_template(
         slots=["<start_of_turn>tool\n{{content}}<end_of_turn>\n<start_of_turn>model\n"]
     ),
     format_prefix=EmptyFormatter(slots=[{"bos_token"}]),
+    stop_words=["<end_of_turn>"],
     mm_plugin=get_mm_plugin(name="paligemma", image_token="<image>"),
 )