[cherry-picked 2.1] Fix backward compatibility layer in backend module

[mthrok]

The PR #3549 re-organized the backend implementations and deprecated the direct access to torchaudio.backend.

The change was supposed to be BC-compatible while issuing a warning to users, but the implementation of module-level __getattr__ was not quite right.

See an issue pyannote/pyannote-audio#1456.

This commit fixes it so that the following imports work;

from torchaudio.backend.common import AudioMetaData

from torchaudio.backend import sox_io_backend
from torchaudio.backend.sox_io_backend import save, load, info

from torchaudio.backend import no_backend
from torchaudio.backend.no_backend import save, load, info

from torchaudio.backend import soundfile_backend
from torchaudio.backend.soundfile_backend import save, load, info

[pytorch-bot]

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@mthrok merged this pull request in 931598c.

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Hey @mthrok.
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