Fix for FileNotFoundError 'output/metrics.json'

Summary:
FileNotFoundError: [Errno 2] No such file or directory: 'output/metrics.json'

Thanks for your contribution!

#3587

With best regards,
Ranuga

Pull Request resolved: #3588

Differential Revision: D31789957

Pulled By: ppwwyyxx

fbshipit-source-id: d5c81a81425cbefef7154aaa52d6e38cfa796385

Author: prdai

detectron2/engine/defaults.py

@@ -240,6 +240,7 @@ def default_writers(output_dir: str, max_iter: Optional[int] = None):
     Returns:
         list[EventWriter]: a list of :class:`EventWriter` objects.
     """
+    PathManager.mkdirs(output_dir)
     return [
         # It may not always print what you want to see, since it prints "common" metrics only.
         CommonMetricPrinter(max_iter),