Fix QA trainer to properly log eval metrics
# What does this PR do?

This PR updates the custom `QuestionAnsweringTrainer` in the examples. Previously, the trainer used `self.log()`, which does not add a prefix `"eval"`. This was a problem especially when reporting metrics to WanDB. Without a prefix, WanDB assumes the metrics come from training data, not evaluation data.

This problem can be solved by simply changing `self.log(metrics)` to `self.log_metrics("eval", metrics)` in both `evaluate()` methods. 

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The prefix is added in the lines above.<|||||>Oh, I guess I used the older version of the QA trainer... 
I feel sorry for bothering you and not fully checking the code. Actually, it was my first public PR...
Anyway, I really appreciate your reply and your efforts to create and maintain this library.
<|||||>No worries! Let us know if the bug persists with the last version.