[fixup/style] requires TF but doesn't say that cleanly
It looks like `make fixup` et al, require tf (and pt), but fail in unexpected way when the requirements are missing:

```
python utils/check_repo.py
Checking all models are properly tested.
Checking all objects are properly documented.
Checking all models are in at least one auto class.
Traceback (most recent call last):
  File "/home/michael/projects/transformers/utils/check_repo.py", line 481, in <module>
    check_repo_quality()
  File "/home/michael/projects/transformers/utils/check_repo.py", line 477, in check_repo_quality
    check_all_models_are_auto_configured()
  File "/home/michael/projects/transformers/utils/check_repo.py", line 290, in check_all_models_are_auto_configured
    all_auto_models = get_all_auto_configured_models()
  File "/home/michael/projects/transformers/utils/check_repo.py", line 253, in get_all_auto_configured_models
    for attr_name in dir(transformers.models.auto.modeling_tf_auto):
  File "/home/michael/projects/transformers/src/transformers/file_utils.py", line 1690, in __getattr__
    raise AttributeError(f"module {self.__name__} has no attribute {name}")
AttributeError: module transformers.models.auto has no attribute modeling_tf_auto
make: *** [Makefile:35 : extra_quality_checks] Erreur 1
```

Thank you, @michaelbenayoun for flagging this

Should we add a small script first that checks all the requirements so that the error is not misleading, but something like: "need to install `pip install -e .[dev]` to develop `transformers`"?

@sgugger, @LysandreJik 
The same thing happens with flax.<|||||>This should take care of all the deps:
```
pip install -e .[dev]
```
Please let us know if it didn't.<|||||>I don't think we need a new script for that. Maybe add the check inside the script that fails (`check_all_models_are_auto_configured`) and issue a warning if not all backends are detected (I don't think we need to error out, since it's unlikely the user will bring changes that break a backend when that backend is not even installed)? I can do this later this evening or tomorrow.<|||||>Also with a bit of some further massaging of `setup.py`'s  `extras`, we could automate this - basically need to be able to load `extras[dev]` outside of `setup.py`, so the check could be to simply import everything that is in `extras[dev]`.<|||||>Note that this specific script only relies on the model backends only, so no need for the whole of dev yet.<|||||>If it's easier - then by all means. 

I just thought that if we already maintain `extras[dev]` then any dev tool could just have that as pre-requisite.