Raise exceptions instead of asserts in src/transformers/models/bart/modeling_flax_[bart, marian, mbart, pegasus].py
Author: killazz67
Pull request: yes

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Fixes  #12789.


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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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Library:

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- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

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HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
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Comments:
I wanted to just change one file but the code quality check forced me to also update the other three files. So that's why I am changing multiple at once. I hope you don't mind. 

@patil-suraj I am confused , do you know why the ci fails to install the dependencies? I thought that it could be a temporary issue with the runner that's why I tried to rerun it, but it somehow is another issue. 

Weird, it seems to be a similar issue to: https://stackoverflow.com/questions/69100275/error-while-downloading-the-requirements-using-pip-install-setup-command-use-2

I have relaunched the tests to see if the issue persists.

@LysandreJik Yeah that issue is really weird and unfortunately it still persists. Maybe i can find something, but i'am not so familiar with the project that I am comfortable changing some dependencies. 

I have traced it back to the release of pip 21.3 today. I have pushed a hotfix on `master`, do you mind rebasing on the `master` branch to include the fix? Thank you!

@LysandreJik i rebased the branch and now it works just fine. I am curious why there are these issues in pip 21.3, did you find anything out?