fix dpo config

Signed-off-by: ashors1 <ashors@nvidia.com>

Author: ashors1

examples/configs/dpo.yaml

@@ -22,7 +22,7 @@ dpo:
 checkpointing:
   enabled: true
   checkpoint_dir: "results/dpo"
-  metric_name: "val:loss"
+  metric_name: "val:validation-default_loss"
   higher_is_better: false
   keep_top_k: 3
   save_period: 50