Fix prompt assembly for llama (#952)

* start at index 0

* add test to check for missing turns

* apply black

* Update test_prompt_tokenizers.py

* Update src/axolotl/monkeypatch/fastchat_conversation_turns.py

Co-authored-by: Motoki Wu <tokestermw@gmail.com>

* fix linting

* apply black

* add more tests for llama/sharegpt

* make logic clearer

---------

Co-authored-by: Motoki Wu <tokestermw@gmail.com>

src/axolotl/monkeypatch/fastchat_conversation_turns.py

@@ -83,14 +83,21 @@ def get_turns(  # pylint: disable=too-many-return-statements
                 yield role + ":", ""
         return
     if self.sep_style == SeparatorStyle.LLAMA2:
-        seps = [self.sep, self.sep2]
         if self.system_message:
+            if self.messages:
+                # For llama, the system message is incorporated into the first human instruction
+                first_role, first_msg = self.messages[0]
+                if first_role == self.roles[0]:
+                    system_prompt += first_msg
+                    self.messages.pop(0)
             yield "", system_prompt
-        else:
-            yield "", "[INST] "
-        for i, (role, message) in enumerate(self.messages[1:]):
+        for i, (role, message) in enumerate(self.messages):
             if message:
-                yield role + " ", message + seps[i % 2]
+                if (i % 2 == 0 and not self.system_message) or (
+                    i % 2 != 0 and self.system_message
+                ):
+                    role = "<s> " + role
+                yield role + " ", message
             else:
                 yield role, ""
         return