Update audio feature extraction tutorial (#2391)

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

Pull Request resolved: #2391

Reviewed By: carolineechen, nateanl

Differential Revision: D36885626

Pulled By: mthrok

fbshipit-source-id: 1306f22ab70ab1e7f74ed7e43bf43150015448b6