improve check for base case

src/axolotl/prompt_strategies/chat_template.py

@@ -223,7 +223,7 @@ class ChatTemplateStrategy(PromptTokenizingStrategy):
     def tokenize_prompt(self, prompt):
         # Old simple legacy behavior that works reliably.
         if (
-            not self.roles_to_train
+            (not self.roles_to_train or self.roles_to_train == ["assistant"])
             and not self.train_on_eos
             and not self.prompter.message_field_training
             and not self.prompter.message_field_training_detail
@@ -487,7 +487,7 @@ def load(tokenizer, cfg, ds_cfg: Optional[Dict[str, Any]] = None, processor=None
     strategy_params = {
         "train_on_inputs": cfg.train_on_inputs,
         "sequence_len": cfg.sequence_len,
-        "roles_to_train": ds_cfg.get("roles_to_train", None),
+        "roles_to_train": ds_cfg.get("roles_to_train", ["assistant"]),
         "train_on_eos": ds_cfg.get("train_on_eos", "turn"),
     }