add tests code

tests/test_utils/test_io_utils.py

+# Copyright (c) OpenMMLab. All rights reserved.
+import os
+from tempfile import TemporaryDirectory
+
+from mmgen.utils import download_from_url
+
+
+def test_download_from_file():
+    img_url = 'https://user-images.githubusercontent.com/12726765/114528756-de55af80-9c7b-11eb-94d7-d3224ada1585.png'  # noqa
+    with TemporaryDirectory() as temp_dir:
+        local_file = download_from_url(url=img_url, dest_dir=temp_dir)
+        assert os.path.exists(local_file)