Allow soft dependencies in the namespace with ImportErrors at use (#7537)

* PoC on RAG

* Format class name/obj name

* Better name in message

* PoC on one TF model

* Add PyTorch and TF dummy objects + script

* Treat scikit-learn

* Bad copy pastes

* Typo

.circleci/config.yml

@@ -248,6 +248,7 @@ jobs:
             - run: isort --check-only examples templates tests src utils
             - run: flake8 examples templates tests src utils
             - run: python utils/check_copies.py
+            - run: python utils/check_dummies.py
             - run: python utils/check_repo.py
     check_repository_consistency:
         working_directory: ~/transformers