Fix a bug in run_qa_no_trainer.py

[dreamgonfly]

What does this PR do?

Fixes a bug in run_qa_no_trainer.py.
In the code, eval_metric dictionary generated from squad metric doesn't have 'exact' key.
KeyError: 'exact' raised during execution.

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[searchivarius]

Hi @dreamgonfly I have a more comprehensive PR that fixes this issue. Turns out that saving of the metrics is tricky, because the unit tests (which fail in your case) actually use SQuAD v2 metric and they have a different name for the exact matching score. Naturally, you were running this code from the command line and it was using the SQuAD v1 metric. This is why your tests are failing, but the command line results are fine (it took me a few hours of head scratching to figure this out):
#16958