Move backend initialization to toplevel

[mthrok]

The backend dispatcher is implemented in torchaudio._backend, while the legacy backend is implemented in torchaudio.backend.

The initialization happen in torchaudio._backend.
This commit moves it to torchaudio.__init__, so that backend and _backend is more independent.

[pytorch-bot]

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❌ 2 New Failures

As of commit 596d383:

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• build (3.8) (gh)
• Build Windows Conda (gh)

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[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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[facebook-github-bot]

@mthrok merged this pull request in 6fb21ab.