GitLab

Summary:
Pull Request resolved: https://github.com/pytorch/audio/pull/3249

- Put ptr member private so that it's more secure and subclasses won't mess with it
- Remove unused `reset` method
- Do not default construct the managed object
  - Introduce helper function for default allocation.
    (for AVFrame and AVPacket as they are allocated in both reader and writer)
  - for others, allocation logics are moved to where it is used.
- Remove unused `pHWBufferRef` attribute from `StreamWriter`.

Reviewed By: hwangjeff

Differential Revision: D44775297

fbshipit-source-id: ff6db528152cd54c1ae398191110c30b9c1e238c