Model confidence 2.0

[Timoeller]

As started in #690 we are developing a way to have better QA model confidence scores.

In this issue we especially focus on developing a method that is easily applicable to out of domain data.

If it doesnt work in OOD data, create concept of what to do next.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 21 days if no further activity occurs.

[Timoeller]

The linked PR 803 was unrelated to Model Confidence, so reopening issue again.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 21 days if no further activity occurs.