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

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

A very similar recent change was done for the variable `config.output_past` -> see PR:https://github.com/huggingface/transformers/pull/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, @patrickvonplaten I want to take up this issue. Can I move forward with it? <|||||>I think this could have side effects for libraries that use `config.output_hidden_states`, so I'm cc'ing @Timoeller and @brandenchan, because this parameter is used in [FARM](https://github.com/deepset-ai/FARM).<|||||>> Hi, @patrickvonplaten I want to take up this issue. Can I move forward with it?

That would be great, feel free to open a PR and do a first model. The PR should be very similar to what was done in PR #3734<|||||>Hi, @patrickvonplaten as I am new here I might take some time to get acquainted with the codebase and come up with a PR. Is it okay?<|||||>Sorry @gaurav-singh1998, I saw this issue was still open so I made a PR for it.<|||||>Okay, no issues @drjosephliu I'll find some other good first issues to solve. <|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
