[flax] fix repo_check (#7914)

* [flax] fix repo_check

Unless, this is actually a problem, this adds `modeling_flax_utils` to ignore list. otherwise currently it expects to have a 'tests/test_modeling_flax_utils.py' for it.
for context please see: https://github.com/huggingface/transformers/pull/3722#issuecomment-712360415

* fix 2 more issues

* merge https://github.com/huggingface/transformers/pull/7919/

.circleci/config.yml

@@ -264,7 +264,7 @@ jobs:
                       - v0.3-{{ checksum "setup.py" }}
             - run: pip install --upgrade pip
             - run: pip install isort
-            - run: pip install .[tf,torch,quality]
+            - run: pip install .[tf,torch,flax,quality]
             - save_cache:
                   key: v0.3-code_quality-{{ checksum "setup.py" }}
                   paths: