drop length column for issues with eval without packing (#1711)

src/axolotl/core/trainer_builder.py

@@ -459,6 +459,8 @@ class AxolotlTrainer(Trainer):
             self.data_collator = (  # pylint: disable=attribute-defined-outside-init
                 self.eval_data_collator
             )
+            if eval_dataset:
+                eval_dataset = eval_dataset.remove_columns(["length"])
             dataloader = super().get_eval_dataloader(eval_dataset)
             self.data_collator = (  # pylint: disable=attribute-defined-outside-init
                 self.train_data_collator