fixed not JSON serializable error in run_qa.py with fp16

[WissamAntoun]

What does this PR do?

Fixed an issue where running the run_qa.py script in a Squad-like dataset with fp16 enabled, would lead to a JSON serialization error:

TypeError: Object of type 'float16' is not JSON serializable

The bug is caused by not converting np.float16 to float on line 209 and 397 in the utils_qa.py file.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section? --> yes
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case. --> no
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings. --> No need for it
• Did you write any new necessary tests? No

Who can review?

@sgugger
Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.

[sgugger]

Thanks a lot for the fix!