Fix DPRReaderTokenizer's attention_mask

[mkserge]

What does this PR do?

This PR fixes an issue with the attention_mask not being properly generated by the DPRReaderTokenizer. Please see issue #9555 for more details.

I added an integration test that checks the DPRReader following a similar example in the test file.

I have some test failures due to

"AttributeError: Can't pickle local object 'measure_peak_memory_cpu.<locals>.MemoryMeasureProcess'"

and

"AttributeError: module 'wandb' has no attribute 'ensure_configured'"

which seem to be unrelated to my code changes.

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?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@LysandreJik and @lhoestq would probably be best positioned to review.

[lhoestq]

LGTM thank you :)

[LysandreJik]

LGTM, thanks @mkserge!

[mkserge]

Thank you both. Should I close the corresponding issue?

[LysandreJik]

Just did! Thanks!