[ConvTemplate] Update deepseek.py (#3118)

* Update deepseek.py

Update system prompts to replicate the official experience as much as possible.

* Fix Pylint

---------

Co-authored-by: Ruihang Lai <ruihangl@cs.cmu.edu>

Author: rankaiyx

python/mlc_llm/conversation_template/deepseek.py

@@ -41,7 +41,9 @@
     Conversation(
         name="deepseek_r1_qwen",
         system_template=f"<｜begin▁of▁sentence｜>{MessagePlaceholders.SYSTEM.value}",
-        system_message="You are a helpful assistant.",
+        system_message="You are Deepseek-R1, an AI assistant created exclusively by the Chinese "
+        "Company DeepSeek. You'll provide helpful, harmless, and detailed responses to all "
+        "user inquiries.",
         roles={"user": "<｜User｜>", "assistant": "<｜Assistant｜>"},
         seps=["", "<｜end▁of▁sentence｜>"],
         role_content_sep="",
@@ -55,7 +57,9 @@
     Conversation(
         name="deepseek_r1_llama",
         system_template=f"<｜begin▁of▁sentence｜>{MessagePlaceholders.SYSTEM.value}",
-        system_message="You are a helpful assistant.",
+        system_message="You are Deepseek-R1, an AI assistant created exclusively by the Chinese "
+        "Company DeepSeek. You'll provide helpful, harmless, and detailed responses to all"
+        " user inquiries.",
         roles={"user": "<｜User｜>", "assistant": "<｜Assistant｜>"},
         seps=["", "<｜end▁of▁sentence｜>"],
         role_content_sep="",