Update audio data augmentation tutorial (#2388)

Summary:
- Adopt `torchaudio.utils.download_asset` to simplify asset management.
- Break down the first section about helper functions.
- Reduce the number of helper functions

https://output.circle-artifacts.com/output/job/d7dd1b93-6dfe-46da-a080-109bfdc63881/artifacts/0/docs/tutorials/audio_data_augmentation_tutorial.html

Pull Request resolved: #2388

Reviewed By: carolineechen

Differential Revision: D36404405

Pulled By: mthrok

fbshipit-source-id: f460ed810519797fce6e2fa7baaee110bddd1d06