add unpack_inputs decorator to mbart tf
Author: Abdelrhman-Hosny
Pull request: yes

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See #16051 


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

@gante 

Also, made a tiny edit to the description of the PR (replaced the word `Fixes` by `See`, as it would close the issue) :)

> Fantastic, thank you so much for the contribution 👍 
> 
> Can you confirm that you've run `RUN_SLOW=1 py.test -vv tests/[model_name]/test_modeling_tf_[model_name].py`? Will merge after your confirmation :)

Yepp, all the tests passed.