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* [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 .