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Summary:
This commit enables ffmpeg-feature build in tests and
binary builds of all platforms.
(Linux/macOS/Windows x conda/wheel)

It also moves the definition of BUILD_FFMPEG env vars to the
top level `config.yml`.

 ---
Manual checking if all the build log contains `libtorchaudio_ffmpeg`.
### binary build
- [x] `binary_linux_conda_py3.7_cpu`
- [x] `binary_linux_conda_py3.7_cu102`
- [x] `binary_linux_wheel_py3.7_cpu`
- [x] `binary_linux_wheel_py3.7_cu102`
- [x] `binary_macos_conda_py3.7_cpu`
- [x] `binary_macos_wheel_py3.7_cpu`
- [x] `binary_windows_conda_py3.7_cpu`
- [x] `binary_windows_conda_py3.7_cu113`
- [x] `binary_windows_wheel_py3.7_cpu`
- [x] `binary_windows_wheel_py3.7_cu113`

### test
- [x] `unittest_linux_cpu_py3.7`
- [x] `unittest_linux_gpu_py3.7`
- [x] `unittest_macos_cpu_py3.7`
- [x] `unittest_windows_cpu_py3.7`
- [x] `unittest_windows_gpu_py3.7`
- [x] `integration test`

Pull Request resolved: https://github.com/pytorch/audio/pull/2140

Reviewed By: hwangjeff

Differential Revision: D33464430

Pulled By: mthrok

fbshipit-source-id: 2c5b72be75d49019bf1599036180d4e56074e46b

This commit changes the build process so that the third party code are fetched automatically when `setup.py` is invoked.

This is to allow
1. `build_sdist` to contain the third party library codes, so that source code distribution created at the time release tag is created is complete.
2. It makes it possible to do `pip install https://github.com/pytorch/audio.git`.
    Example:
    ```
    !pip install 'cmake>=3.18' ninja
    !pip install --verbose git+https://github.com/pytorch/audio.git@auto-source
    ```