Update llama_chat_format.py (#869)

* Update llama_chat_format.py

properly formal llama2 with first-message prompt embedded

* Update llama_chat_format.py

Author: earonesty

llama_cpp/llama_chat_format.py

@@ -73,13 +73,16 @@ def _map_roles(
 
 
 def _format_llama2(
-    system_message: str, messages: List[Tuple[str, Optional[str]]], sep: str
+    system_message: str, messages: List[Tuple[str, Optional[str]]], sep: str, sep2: str
 ) -> str:
     """Format the prompt with the llama2 style."""
+    seps = [sep, sep2]
     ret = system_message + sep
-    for role, message in messages:
-        if message:
-            ret += role + message + " "
+    for i, (role, message) in enumerate(messages):
+        if system_message and i == 0:
+            ret += message + seps[i % 2]
+        elif message:
+            ret += role + message + " " + seps[i % 2]
         else:
             ret += role + " "
     return ret
@@ -324,19 +327,20 @@ def get_chat_format(name: str):
         )
 
 
+# see https://github.com/huggingface/transformers/blob/main/src/transformers/models/llama/tokenization_llama.py
+# system prompt is "embedded" in the first message
 @register_chat_format("llama-2")
 def format_llama2(
     messages: List[llama_types.ChatCompletionRequestMessage],
     **kwargs: Any,
 ) -> ChatFormatterResponse:
-    _system_template = "[INST] <<SYS>>\n{system_message}\n<</SYS>>\n\n"
-    _roles = dict(user="[INST]", assistant="[/INST]")
-    _sep = "\n\n"
-    system_message = _get_system_message(messages)
-    system_message = _system_template.format(system_message=system_message)
+    _system_template = "<s>[INST] <<SYS>>\n{system_message}\n<</SYS>>"
+    _roles = dict(user="<s>[INST]", assistant="[/INST]")
     _messages = _map_roles(messages, _roles)
-    _messages.append((_roles["assistant"], None))
-    _prompt = _format_llama2(system_message, _messages, _sep)
+    system_message = _get_system_message(messages)
+    if system_message:
+        system_message = _system_template.format(system_message=system_message)
+    _prompt = _format_llama2(system_message, _messages, " ", "</s>") + "[/INST]"
     return ChatFormatterResponse(prompt=_prompt)