fix errors for lm_finetuning examples

examples/lm_finetuning/finetune_on_pregenerated.py

@@ -325,6 +325,7 @@ def main():
                     global_step += 1
 
     # Save a trained model
+    if torch.distributed.get_rank() == 0:
         logging.info("** ** * Saving fine-tuned model ** ** * ")
         model_to_save = model.module if hasattr(model, 'module') else model  # Only save the model it-self