Report Tune metrics in final evaluation
# What does this PR do?

This PR makes Ray Tune's tuning objective function report all metrics, not just the objective, in the final evaluation step. It also gets rid of the (unnecessary) return value. With these changes the training objective is fully compatible with Ray Tune's recently introduced strict metric checking.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dimiss 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?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to the 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?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.

@sgugger 
Just to be sure, is it also backward compatible with older versions of Ray Tune?<|||||>Yes! Return values were never required, and in fact disregarded until recently.

On October 1, 2020 2:38:00 PM GMT+01:00, Sylvain Gugger <notifications@github.com> wrote:
>Just to be sure, is it also backward compatible with older versions of
>Ray Tune?
>
>-- 
>You are receiving this because you authored the thread.
>Reply to this email directly or view it on GitHub:
>https://github.com/huggingface/transformers/pull/7507#issuecomment-702141017
<|||||>Thanks for the fix then!