GitLab

Summary:
* Refactor _extension module so that
  * the implementation of initialization logic and its execution are separated.
    * logic goes to `_extension.utils`
    * the execution is at `_extension.__init__`
    * global variables are defined and modified in `__init__`.
* Replace `is_sox_available()` with `_extension._SOX_INITIALIZED`
* Replace `is_kaldi_available()` with `_extension._IS_KALDI_AVAILABLE`
* Move `requies_sox()` and `requires_kaldi()` to break the circular dependency among `_extension` and `_internal.module_utils`.
* Merge the sox-related initialization logic in `_extension.utils` module.

Pull Request resolved: https://github.com/pytorch/audio/pull/2968

Reviewed By: hwangjeff

Differential Revision: D42387251

Pulled By: mthrok

fbshipit-source-id: 0c3245dfab53f9bc1b8a83ec2622eb88ec96673f