Use HF_HUB_OFFLINE + fix has_file in offline mode (#31016)

* Fix has_file in offline mode

* harmonize env variable for offline mode

* Switch to HF_HUB_OFFLINE

* fix test

* revert test_offline to test TRANSFORMERS_OFFLINE

* Add new offline test

* merge conflicts

* docs

tests/test_configuration_utils.py

@@ -12,7 +12,6 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
 import json
 import os
 import shutil