Adding should_continue check for retraining (#3509)

examples/run_language_modeling.py

@@ -624,6 +624,7 @@ def main():
         and os.listdir(args.output_dir)
         and args.do_train
         and not args.overwrite_output_dir
+        and not args.should_continue
     ):
         raise ValueError(
             "Output directory ({}) already exists and is not empty. Use --overwrite_output_dir to overcome.".format(