Refactor arg mapping in ffmpeg save function

[mthrok]

The arguments of TorchAudio's save function ("format", "bits_per_sample" and "encoding")
are not one-to-one mapping to the arguments of FFmpeg encoding.

For example, to use vorbis codec, FFmpeg expects "ogg" container/extension with "vorbis"
encoder. It does not recognize "vorbis" extension like TorchAudio (libsox) does.

This commit refactors the logic to parse/map the arguments.

As a result it now properly works with vorbis and mp3 extension.

[pytorch-bot]

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❌ 4 New Failures

As of commit 90061a1:

NEW FAILURES - The following jobs have failed:

• build (3.8) (gh)
• Build M1 Conda (gh)
• pytorch/audio / wheel-py3_8-cuda11_8 (gh)
• tests / macos-job (gh)

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[facebook-github-bot]

@mthrok merged this pull request in b99e5f4.

[github-actions]

Hey @mthrok.
You merged this PR, but labels were not properly added. Please add a primary and secondary label (See https://github.com/pytorch/audio/blob/main/.github/process_commit.py).

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