[ConvTemplate] Update Gemma template with <bos> (#2722)

This commit adds `<bos>` to the gemma's conversation template.

Author: MasterJH5574

python/mlc_llm/conversation_template/gemma.py

@@ -8,7 +8,7 @@
 ConvTemplateRegistry.register_conv_template(
     Conversation(
         name="gemma_instruction",
-        system_template=f"{MessagePlaceholders.SYSTEM.value}",
+        system_template=f"<bos>{MessagePlaceholders.SYSTEM.value}",
         system_message="",
         roles={"user": "<start_of_turn>user", "assistant": "<start_of_turn>model"},
         seps=["<end_of_turn>\n"],