Fix it to run properly even if without --do_train param.

It was modified similar to `run_classifier.py`, and Fixed to run properly even if without `--do_train` param.

examples/run_squad.py

@@ -782,7 +782,7 @@ def main():
             raise ValueError(
                 "If `do_predict` is True, then `predict_file` must be specified.")
 
-    if os.path.exists(args.output_dir) and os.listdir(args.output_dir):
+    if os.path.exists(args.output_dir) and os.listdir(args.output_dir) and args.do_train:
         raise ValueError("Output directory () already exists and is not empty.")
     os.makedirs(args.output_dir, exist_ok=True)
 
@@ -916,6 +916,7 @@ def main():
     # Save a trained model
     model_to_save = model.module if hasattr(model, 'module') else model  # Only save the model it-self
     output_model_file = os.path.join(args.output_dir, "pytorch_model.bin")
+    if args.do_train:
         torch.save(model_to_save.state_dict(), output_model_file)
 
     # Load a trained model that you have fine-tuned