Mark Places365 and SBU download tests as xfail(#6389)

* disable download test for Places365

* add xfail test for broken download servers

* add more comments

* mark SBU download as failing
Co-authored-by: Nicolas Hug <contact@nicolas-hug.com>

test/test_datasets_download.py

@@ -433,7 +433,6 @@ def make_parametrize_kwargs(download_configs):
 @pytest.mark.parametrize(
     **make_parametrize_kwargs(
         itertools.chain(
-            places365(),
             caltech101(),
             caltech256(),
             cifar10(),
@@ -448,7 +447,6 @@ def make_parametrize_kwargs(download_configs):
             omniglot(),
             phototour(),
             sbdataset(),
-            sbu(),
             semeion(),
             stl10(),
             svhn(),
@@ -461,6 +459,31 @@ def make_parametrize_kwargs(download_configs):
     )
 )
 def test_url_is_accessible(url, md5):
+    """
+    If you see this test failing, find the offending dataset in the parametrization and move it to
+    ``test_url_is_not_accessible`` and link an issue detailing the problem.
+    """
+    retry(lambda: assert_url_is_accessible(url))
+
+
+@pytest.mark.parametrize(
+    **make_parametrize_kwargs(
+        itertools.chain(
+            places365(),  # https://github.com/pytorch/vision/issues/6268
+            sbu(),  # https://github.com/pytorch/vision/issues/6390
+        )
+    )
+)
+@pytest.mark.xfail
+def test_url_is_not_accessible(url, md5):
+    """
+    As the name implies, this test is the 'inverse' of ``test_url_is_accessible``. Since the download servers are
+    beyond our control, some files might not be accessible for longer stretches of time. Still, we want to know if they
+    come back up, or if we need to remove the download functionality of the dataset for good.
+
+    If you see this test failing, find the offending dataset in the parametrization and move it to
+    ``test_url_is_accessible``.
+    """
     retry(lambda: assert_url_is_accessible(url))