Replace `config.output_attentions` parameter with function argument `output_attentions`
# 🚀 Feature request

Currently the user has to decide whether the model should output the attentions when she/he creates the config of a model: config.output_attentions = True/False. It would be nice if the user can decide this when calling the models `forward()` / `call()` with a flag `output_attentions`. This should be done for all TF and PT models that can output attentions.

A very similar recent change was done for the variable `config.output_past` -> see PR:#3734

## Motivation

The user has more flexibility when the hidden states should be output or not.

## Your contribution

If someone feels like contributing to the library, this would be a great first PR. I'm very happy to guide the contributor through the PR!

Hi, I would like to work on this issue.<|||||>That's great :-) Do you want to open a PR and do the changes analogous to PR: #3734 ? <|||||>Is this still be worked on? If not, I'd be happy to make a first contribution here<|||||>First PR first serve ;-) Still an open issue<|||||>Any tips on how I should proceed? I was thinking of following the changes made for `config.output_past` (01c37dc), but for `config.output_attentions` instead.<|||||>Oh sorry @drjosephliu didn't notice the comment as I was working on it earlier today, my apologies 😞 <|||||>Hey @patrickvonplaten, i noticed this issue has been closed. Any updates on what changes were made and any updates to the PR i still need to make?<|||||>Hey @drjosephliu, I will take a closer look at your PR tomorrow :-) 