feat: Add Gemma3 chat handler (#1976)

llama_cpp/llama_chat_format.py

@@ -3519,6 +3519,57 @@ def __call__(self, **kwargs):
         return super().__call__(**kwargs)
 
 
+class Gemma3ChatHandler(Llava15ChatHandler):
+    # Chat Format:
+    # '<bos><start_of_turn>user\n{system_prompt}\n\n{prompt}<end_of_turn>\n<start_of_turn>model\n'
+
+    DEFAULT_SYSTEM_MESSAGE = None
+
+    CHAT_FORMAT = (
+        "{% if messages[0]['role'] == 'system' %}"
+        "{% if messages[0]['content'] is string %}"
+        "{% set first_user_prefix = messages[0]['content'] + '\n\n' %}"
+        "{% else %}"
+        "{% set first_user_prefix = messages[0]['content'][0]['text'] + '\n\n' %}"
+        "{% endif %}"
+        "{% set loop_messages = messages[1:] %}"
+        "{% else %}"
+        "{% set first_user_prefix = \"\" %}"
+        "{% set loop_messages = messages %}"
+        "{% endif %}"
+        "{% for message in loop_messages %}"
+        "{% if (message['role'] == 'user') != (loop.index0 % 2 == 0) %}"
+        "{{ raise_exception(\"Conversation roles must alternate user/assistant/user/assistant/...\") }}"
+        "{% endif %}"
+        "{% if (message['role'] == 'assistant') %}"
+        "{% set role = \"model\" %}"
+        "{% else %}"
+        "{% set role = message['role'] %}"
+        "{% endif %}"
+        "{{ '<start_of_turn>' + role + '\n' + (first_user_prefix if loop.first else \"\") }}"
+        "{% if message['content'] is string %}"
+        "{{ message['content'] | trim }}"
+        "{% elif message['content'] is iterable %}"
+        "{% for item in message['content'] %}"
+        "{% if item['type'] == 'image_url' and item['image_url'] is string %}"
+        "{{ '\n\n' + item['image_url'] + '\n\n' }}"
+        "{% elif item['type'] == 'image_url' and item['image_url'] is mapping %}"
+        "{{ '\n\n' + item['image_url']['url'] + '\n\n' }}"
+        "{% elif item['type'] == 'text' %}"
+        "{{ item['text'] | trim }}"
+        "{% endif %}"
+        "{% endfor %}"
+        "{% else %}"
+        "{{ raise_exception(\"Invalid content type\") }}"
+        "{% endif %}"
+        "{{ '<end_of_turn>\n' }}"
+        "{% endfor %}"
+        "{% if add_generation_prompt %}"
+        "{{ '<start_of_turn>model\n' }}"
+        "{% endif %}"
+    )
+
+
 @register_chat_completion_handler("chatml-function-calling")
 def chatml_function_calling(
     llama: llama.Llama,