feat: do not find turn indices if turn is not trainable (#2696)

* feat: do not find turn indices if turn is not trainable

* fix: handle edge case where train on eos/eot is all

* fix: improve warning message

src/axolotl/prompt_strategies/chat_template.py

@@ -424,6 +424,20 @@ class ChatTemplateStrategy(PromptTokenizingStrategy):
 
             LOG.debug(f"Should train: {should_train}")
 
+            # turn not trainable, skip having to find the turn indices
+            # unless last turn and train_on_eos/train_on_eot is all
+            if not should_train and (
+                self.train_on_eos != "all" and self.train_on_eot != "all"
+            ):
+                if index == len(turns) - 1:
+                    LOG.warning(
+                        "Last turn is not trainable, skipping having to find the turn indices. "
+                        "This may cause incorrect last EOT/EOS token to be unmasked."
+                        "This is likely a dataset design issue. Please ensure last turn is trainable."
+                    )
+
+                continue
+
             turn_start_idx, turn_end_idx = self.find_turn(turns=turns, turn_idx=index)
 
             LOG.debug(f"Turn indices: start={turn_start_idx}, end={turn_end_idx}")