Add unpack_inputs decorator for ctrl
# What does this PR do?

Add the `unpack_inputs` decorator for `ctrl`. It also replaces the `past` parameters in models in and output by `past_key_values` as there was an irregularity in the naming that caused an error with the new input processing.

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Fixes # (issue)
#16051 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@gante


## Tests
I ran `RUN_SLOW=1 py.test -vv tests/ctrl/test_modeling_tf_ctrl.py` but it only came to around 69% and failed the pre-trained model test, because it is too big for my local machine to test it.
_The documentation is not available anymore as the PR was closed or merged._<|||||>Going to run the slow tests locally, to confirm they pass<|||||>Can confirm that they pass, merging 👍 