Add template for adding flax models
# What does this PR do?

Fixes #12440

## From Patrick
@LysandreJik @sgugger Previously both the PT and TF encoder-decoder templates tests weren't run because the test name didn't match the "*template*" regex - I've fixed that here as pointed out in the comment below. 

@cccntu added both FlaxBERT and FlaxBART templates and we've added two tests to make sure those work as expected. 

The PR should be good for review now :-)
It's mostly done. I can successfully create a new model with flax. See #12454
TODO:
* Finish modeling_flax for encoder-decoder architecture
* Tests are mapped from tf to np, need fixes.
* Refactor coockie-cutter workflow, make flax blend in with pytorch, tensorflow.
* ??<|||||>I think only the test for template is failing now, can someone take a quick look? Thanks!
@patil-suraj @patrickvonplaten @LysandreJik <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Maybe I should rebase and open another PR?<|||||>Sorry for being so late on this one! I'll take a look tomorrow!<|||||>yaaaay - finally all the tests are green! @cccntu - amazing work. I've done a lot of small fixes that are quite time-consuming and tedious to make everything pass, but the main work was all done by you - thanks a mille!<|||||>Thank you very much @patrickvonplaten! 🤗
This is my biggest open source contribution so far, so I really appreciate you help fixing so many stuff and writing the test! (I merely copied the tf version and did some simple substitution.)

I am curious how do you run the template tests? It was confusing to run them locally because the it would overwrite the tracked files and leave unused files, making the work tree messy. I guess `commit -> test -> clean, reset --hard` would work, but I always fear that I would accidentally delete the wrong thing.