grab sys prompt too from dataset (#2397) [skip ci]

* grab sys prompt too from dataset

* chore: add field_system to docs

---------

Co-authored-by: NanoCode012 <nano@axolotl.ai>

src/axolotl/prompt_strategies/chat_template.py

@@ -33,6 +33,7 @@ class ChatTemplatePrompter(Prompter):
         message_field_training: Optional[str] = None,
         message_field_training_detail: Optional[str] = None,
         field_messages: str = "messages",
+        field_system: str = "system",
         roles: Optional[Dict[str, List[str]]] = None,
         drop_system_message: bool = False,
     ):
@@ -62,6 +63,7 @@ class ChatTemplatePrompter(Prompter):
         self.message_field_training = message_field_training
         self.message_field_training_detail = message_field_training_detail
         self.field_messages = field_messages
+        self.field_system = field_system
         self.tokenizer = tokenizer
         self.processor: Optional[ProcessorMixin] = processor
         self.chat_template = chat_template
@@ -488,6 +490,17 @@ class ChatTemplateStrategy(PromptTokenizingStrategy):
 
     def get_conversation_thread(self, prompt):
         turns = []
+
+        possible_sys_turn = self.transform_message(
+            prompt[self.prompter.field_messages][0]
+        )
+        if (
+            possible_sys_turn["role"] != "system"
+            and self.prompter.field_system in prompt
+        ):
+            turn = {"role": "system", "content": prompt[self.prompter.field_system]}
+            turns.append(turn)
+
         for message in prompt[self.prompter.field_messages]:
             transformed_message = self.transform_message(message)