[setup] update/add setup targets
updating pip targets

* [x] adds `tokenizer`
* [x] adds `docs` to `dev`, since we need to have the tools to run `make docs`
* [x] adds `flax` to `dev`, since we need to have the libs to run flax tests - except when on windows - it skips it then
* [x] brings `all` up-to-date


@sgugger 
Guys, since this is not code but a bunch of definitions please make direct suggestions that can be merged.  Thank you. 

I originally just wanted to add `docs` and `flax` to `dev` and had no idea about all the other needs.<|||||>I can't make a suggestion that moves stuff around in the file. I can push a commit to your branch if you want, but that's the best I can do.<|||||>I understand. I just don't know to work in this fashion. If you give me a spec I can work to implement it. Otherwise let's just merge this and subsequent PRs can do further improvements. <|||||>Sure, I'll add my comments in a separate PR. Thanks for doing this one!
