Fix DPRReaderTokenizer's attention_mask
# 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).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? 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](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

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

Thank you both. Should I close the corresponding issue?<|||||>Just did! Thanks!