Long QuestionAnsweringPipeline fix.
Author: Narsil
Pull request: yes

# What does this PR do?

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This handles CLS index even on splitted QA context.

TODO update the tests for one that actually showcases the bug.

Fixes #16769


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Comments:
@LysandreJik  This is not fully ready yet as the test does not really check that the bug is fixed. Do you have any ideas on how to check that behavior ? (Previous code was just silently not doing anything since the np.arrays are an array of `np.list(..)` in the splitted case.)

Edit: It's not covered by a slow test. I am hesitant to remove the fast test because it does not cover this.

_The documentation is not available anymore as the PR was closed or merged._

I'll remove the fast test, don't want to leave a false sense of security here.