[MRG] Backend for optim (#282)

* Backend for optim

* Bug solve

* Doc update

* backend tests now with fixture

* Unused imports removed

* Docs

* Docs

* Docs

* Outer product backend docs

* Prettier docs

* Pep8

* Mistakes corrected

Co-authored-by: Rémi Flamary <[email protected]>

Author: ncassereau