Fix default value for output_image_format

It should be None or won't read from parameters file

mdai_utils/download_annotations.py

@@ -1015,7 +1015,7 @@ def get_download_parser():
     parser.add_argument(
         "--output_image_format",
         type=str,
-        default="nrrd",
+        default=None,
         help="""Output image format for the volumes.""",
     )