GitLab

Summary:
This commits add tutorial for oscillator_bank and adsr_envelope, which will be a basis for DDSP.

 - [Review here](https://output.circle-artifacts.com/output/job/cf1d3001-88e5-418b-8cf8-ae22b4445dba/artifacts/0/docs/tutorials/oscillator_tutorial.html)

Pull Request resolved: https://github.com/pytorch/audio/pull/2862

Reviewed By: carolineechen

Differential Revision: D41559503

Pulled By: mthrok

fbshipit-source-id: 3f1689186db7d246de14f228fc2f91bf37db98cd