fixed not JSON serializable error in run_qa.py with fp16
Author: WissamAntoun
Pull request: yes

# 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.

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