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Summary:
This commits move helper functions/definitions around so that better locality of logics are achieved.

## Detail

`ffmpeg.[h|cpp]` implements classes that convert FFmpeg structures into RAII semantics.
Initially it these classes included the construction logic in their constructors, but such logics were
extracted to factory functions in https://github.com/pytorch/audio/issues/2373.

Now the reason why the factory functions stayed in `ffmpeg.[h|cpp]` was because the logic for
the initialization and  clean-up of AVDictionary class was only available in `ffmpeg.cpp`.

Now AVDictionary class handling is properly defined in https://github.com/pytorch/audio/issues/2507, the factory functions, which are not
that reusable better stay with the implementation that use them.

This makes `ffmpeg.h` lean and clean, makes it easier to see what can be reused.

Pull Request resolved: https://github.com/pytorch/audio/pull/2512

Reviewed By: hwangjeff

Differential Revision: D37477592

Pulled By: mthrok

fbshipit-source-id: 8c1b5059ea5f44649cc0eb1f82d1a92877ef186e