Update Loss calculation in prediction_step
In `predition_step` function, loss is calculated using `_compute_loss` function. If the `compute_loss` function is overridden by a customized loss function, the evaluation will still calculate the original loss. I think it is more interesting to use `compute_loss` function rather than `_compute_loss` function in `prediction_step`.

# What does this PR do?


## Before submitting
- [ x ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Did you make sure to update the documentation with your changes? I think that no need to update documentation.


## Who can review?

Library:

- text generation: @patrickvonplaten
- trainer: @sgugger
- pipelines: @LysandreJik
No this won't work with label smoothing, so we can't accept that change.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.