Fix incorrect reference to member output_dir (nutonomy#45)

Doing the same here, so that run_eval() works even when not called as __main__.

python-sdk/nuscenes/eval/nuscenes_eval.py

@@ -284,7 +284,7 @@ def run_eval(self) -> Dict:
             'weighted_sum': weighted_sum,
             'eval_time': eval_time
         }
-        with open(os.path.join(output_dir, 'metrics.json'), 'w') as f:
+        with open(os.path.join(self.output_dir, 'metrics.json'), 'w') as f:
             json.dump(all_metrics, f, indent=2)
 
         return all_metrics