fix the error when using pyarrow for raw_audio_dataset (#3561)
fix the error when using pyarrow for raw_audio_dataset (#3561)

Summary:
# Before submitting

- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [X] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/master/CONTRIBUTING.md)?
- [ ] Did you make sure to update the docs?
- [X] Did you write any new necessary tests?

## What does this PR do?

fix the error when using pyarrow for raw_audio_dataset, Currently there is an error `TypeError: join() argument must be str, bytes, or os.PathLike object, not 'StringScalar'`. In this PR I just convert the type to string in `__getitem__()` for `os.path.join()`

## PR review
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If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

## Did you have fun?
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Pull Request resolved: https://github.com/pytorch/fairseq/pull/3561

Reviewed By: jmp84

Differential Revision: D28594620

Pulled By: kahne

fbshipit-source-id: fd2daa992df85ac0919ba30fa9afa67a0c89d956

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq