fix: Update run_qa.py to work with deepset/germanquad
Author: sjrl
Pull request: yes

# What does this PR do?
This updates the `run_qa.py` script in the `examples/pytorch/question-answering` folder to work with the `deepset/germanquad` dataset. The script expects the ID field for each example in the dataset to be a string type, but the previously mentioned dataset stores the ID field as an int. So I added a `str` call on the id field to make sure any integer IDs are converted into the expected string format for squad datasets when using the `evaluate` library. This is relevant for https://huggingface.co/datasets/deepset/germanquad which stores the IDs of each example with an integer ID.

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Fixes #N/A


## Before submitting
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## Who can review?

Hey @sgugger I would appreciate your review on this PR! I tagged you based on the recommendation of the PR template since you are listed as the maintainer of the pytorch examples.


Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23225). All of your documentation changes will be reflected on that endpoint.

Could you explain what fails when leaving the code as is?

Sure! I get a `pyarrow` error that the format of `predictions` does not match the defined schema for `predictions` which expects the ID field to be of type string. I'm a bit busy at the moment, but later I can reproduce the error and copy paste the message here. 

That's is a bit weird as we only use pyarrow through `dataset` but this is after the dataset creation.

Is pyarrow also used in the Evaluation library for computing the squad_v2 metrics? It seemed the schema enforcement was for the predictions format because germanquad has its own schema in its dataset repo. 

Ah good catch! Yes I get the issue now.