Fix QA sample
# What does this PR do?

Try to address the issue about `PT_QUESTION_ANSWERING_SAMPLE` discussed in the following 2 comments:

https://github.com/huggingface/transformers/pull/16523#issuecomment-1088635272

https://github.com/huggingface/transformers/pull/16523#issuecomment-1090346578
@patrickvonplaten @sgugger  

I would like to have a feedback to see if this is in good direction.

@sgugger `make style` will break the line into 2 lines. But when replacing the `qa_target_start_index` and `qa_target_end_index` by `14` & `15`, it actually fits in one line. Should I use a shorter name like `qa_target_start_idx` or `qa_start_index` for example?<|||||>cc @vumichien and @bhadreshpsavani to keep them updated<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>I should do the same for `TF_QUESTION_ANSWERING_SAMPLE`. Currently just to have some feedbacks from you if any :-)<|||||>I also changed `TF_QUESTION_ANSWERING_SAMPLE`. 

Ran locally for `Roberta` and `TFRoberta`, and their doctest pass.

Ready for @sgugger to give a final look :-)<|||||>@vumichien @bhadreshpsavani 

Could you pull the latest change into the master/main branch of your local clone, rebase your working branch for this sprint, and check if the QA doc example can now pass the doctest, please? Thanks a lot!