Reorganize file utils (#16264)

* Split file_utils in several submodules

* Fixes

* Add back more objects

* More fixes

* Who exactly decided to import that from there?

* Second suggestion to code with code review

* Revert wront move

* Fix imports

* Adapt all imports

* Adapt all imports everywhere

* Revert this import, will fix in a separate commit

tests/trainer/test_trainer.py

@@ -39,7 +39,6 @@ from transformers import (
     is_torch_available,
     logging,
 )
-from transformers.file_utils import WEIGHTS_NAME, is_apex_available
 from transformers.testing_utils import (
     ENDPOINT_STAGING,
     PASS,
@@ -66,6 +65,7 @@ from transformers.testing_utils import (
 )
 from transformers.trainer_utils import PREFIX_CHECKPOINT_DIR
 from transformers.training_args import OptimizerNames
+from transformers.utils import WEIGHTS_NAME, is_apex_available
 from transformers.utils.hp_naming import TrialShortNamer