Fix mpt prompt template
diff --git a/fastchat/conversation.py b/fastchat/conversation.py
index c2235d7d7b..09778b27c9 100644
--- a/fastchat/conversation.py
+++ b/fastchat/conversation.py
@@ -19,6 +19,7 @@ class SeparatorStyle(Enum):
     RWKV = auto()
     PHOENIX = auto()
     ROBIN = auto()
+    CHATML = auto()
 
 
 @dataclasses.dataclass
@@ -127,6 +128,14 @@ def get_prompt(self) -> str:
                 else:
                     ret += role + ":\n"
             return ret
+        elif self.sep_style == SeparatorStyle.CHATML:
+            ret = "" if self.system == "" else self.system + self.sep + "\n"
+            for role, message in self.messages:
+                if message:
+                    ret += role + "\n" + message + self.sep + "\n"
+                else:
+                    ret += role + "\n"
+            return ret
         else:
             raise ValueError(f"Invalid style: {self.sep_style}")
 
@@ -474,12 +483,11 @@ def get_conv_template(name: str) -> Conversation:
 - You are a helpful assistant chatbot trained by MosaicML.
 - You answer questions.
 - You are excited to be able to help the user, but will refuse to do anything that could be considered harmful to the user.
-- You are more than just an information source, you are also able to write poetry, short stories, and make jokes.
-""",
+- You are more than just an information source, you are also able to write poetry, short stories, and make jokes.""",
         roles=("<|im_start|>user", "<|im_start|>assistant"),
         messages=(),
         offset=0,
-        sep_style=SeparatorStyle.ADD_NEW_LINE_SINGLE,
+        sep_style=SeparatorStyle.CHATML,
         sep="<|im_end|>",
         stop_token_ids=[50278, 0],
     )
@@ -490,12 +498,11 @@ def get_conv_template(name: str) -> Conversation:
     Conversation(
         name="mpt-30b-chat",
         system="""<|im_start|>system
-A conversation between a user and an LLM-based AI assistant. The assistant gives helpful and honest answers.
-""",
+A conversation between a user and an LLM-based AI assistant. The assistant gives helpful and honest answers.""",
         roles=("<|im_start|>user", "<|im_start|>assistant"),
         messages=(),
         offset=0,
-        sep_style=SeparatorStyle.ADD_NEW_LINE_SINGLE,
+        sep_style=SeparatorStyle.CHATML,
         sep="<|im_end|>",
         stop_token_ids=[50278, 0],
     )

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## Why are these changes needed?

Fix mpt prompt template

<!-- Please give a short summary of the change and the problem this solves. -->

## Related issue number (if applicable)

<!-- For example: "Closes #1234" -->

#1783 

## Checks

- [x] I've run `format.sh` to lint the changes in this PR.
- [x] I've included any doc changes needed.
- [ ] I've made sure the relevant tests are passing (if applicable).
