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* [code quality] merge style and quality targets

Any reason why we don't run `flake8` in `make style`? I find myself needing to run `make style` and `make quality` all the time, but I need the latter just for the last 2 checks. Since we have no control over the source code why bother with separating checking and fixing - let's just have one target that fixes and then performs the remaining checks, as we know the first two have been done already.

This PR suggests to merge the 2 targets into one efficient target.

I will edit the docs if this change resonates with the team.

* move checks into style, re-use target

* better name

* add fixup target

* document new target

* fix confused flake

We run `black  --target-version py35 ...` but flake8 doesn't know that, so currently with py38 flake8 fails suggesting that black should have reformatted 63 files. Indeed if I run:

```
black --line-length 119 --target-version py38 examples templates tests src utils
```
it indeed reformats 63 files.

The only solution I found is to create a black config file as explained at https://github.com/psf/black#configuration-format, which is what this PR adds.

Now flake8 knows that py35 is the standard and no longer gets confused regardless of the user's python version.

* adjust the other files that will now rely on black's config file

* Copy code from Bert to Roberta and add safeguard script

* Fix docstring

* Comment code

* Formatting

* Update src/transformers/modeling_roberta.py
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Add test and fix bugs

* Fix style and make new comand
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* [doc] multiple corrections to "Summary of the tasks"

* add a new "docs" target to validate docs and document it

* fix mixup

* Run new isort

* More changes

* Update CI, CONTRIBUTING and benchmarks

* Add a script to check all models are tested and documented

* Apply suggestions from code review
Co-authored-by: Kevin Canwen Xu <canwenxu@126.com>

* Address comments
Co-authored-by: Kevin Canwen Xu <canwenxu@126.com>

This should stabilize formatting.

Also provide shortcuts in a Makefile.

This prevents transformers from being importable simply because the CWD
is the root of the git repository, while not being importable from other
directories. That led to inconsistent behavior, especially in examples.

Once you fetch this commit, in your dev environment, you must run:

    $ pip uninstall transformers
    $ pip install -e .