Remove speed metrics from default compute objective [WIP]
# What does this PR do?

This PR removes speed metrics (e.g. `eval_runtime`) from the default compute objective (`default_compute_objective`). `default_compute_objective` is used when no `compute_objective` is passed to `Trainer.hyperparameter_search`. `Trainer` adds speed metrics such as `eval_runtime` and `eval_samples_per_second` to the metrics and `default_compute_objective` returns the sum of metrics as the objective so these speed metrics will be included in the objective. 

I still need to add unit test for `default_compute_objective` to avoid having such metrics in the objective in the future. 

## 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?
- [ ] 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?
@sgugger 

Oh boy, that is rather bad! Thanks a lot for fixing this!
Did you want to add the test in this PR?<|||||>> Oh boy, that is rather bad! Thanks a lot for fixing this!
> Did you want to add the test in this PR?

I can also create a follow up PR if you want to merge this asap. I can't implement the test cases right away. Maybe in ~4 days. @sgugger<|||||>In that case maybe a follow-up PR, this fix is needed badly so I will merge. Thanks again!