Make Transformers use cache files when hf.co is down (#16362)

* Make Transformers use cache files when hf.co is down

* Fix tests

* Was there a random circleCI failure?

* Isolate patches

* Style

* Comment out the failure since it doesn't fail anymore

* Better comment

tests/test_configuration_common.py

@@ -20,7 +20,7 @@ import shutil
 import sys
 import tempfile
 import unittest
-import unittest.mock
+import unittest.mock as mock
 from pathlib import Path
 
 from huggingface_hub import Repository, delete_repo, login
@@ -304,6 +304,22 @@ class ConfigTestUtils(unittest.TestCase):
                 f"pick another value for them: {', '.join(keys_with_defaults)}."
             )
 
+    def test_cached_files_are_used_when_internet_is_down(self):
+        # A mock response for an HTTP head request to emulate server down
+        response_mock = mock.Mock()
+        response_mock.status_code = 500
+        response_mock.headers = []
+        response_mock.raise_for_status.side_effect = HTTPError
+
+        # Download this model to make sure it's in the cache.
+        _ = BertConfig.from_pretrained("hf-internal-testing/tiny-random-bert")
+
+        # Under the mock environment we get a 500 error when trying to reach the model.
+        with mock.patch("transformers.utils.hub.requests.head", return_value=response_mock) as mock_head:
+            _ = BertConfig.from_pretrained("hf-internal-testing/tiny-random-bert")
+            # This check we did call the fake head request
+            mock_head.assert_called()
+
 
 class ConfigurationVersioningTest(unittest.TestCase):
     def test_local_versioning(self):