Add backend module and isolate backend-related functionalities

[mthrok]

This PR introduces torchaudio.backend module so that we can add proper decoupling and abstraction to audio backend.

Before:

torchaudio
├── _backend.py
├── _soundfile_backend.py
├── _sox_backend.py
...

After:

torchaudio
├── backend
│   ├── __init__.py
│   ├── soundfile_backend.py
│   ├── sox_backend.py
│   └── utils.py
...

[codecov]

Codecov Report

Merging #700 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #700      +/-   ##
==========================================
+ Coverage   89.23%   89.28%   +0.04%     
==========================================
  Files          23       24       +1     
  Lines        2341     2342       +1     
==========================================
+ Hits         2089     2091       +2     
+ Misses        252      251       -1     

Impacted Files	Coverage Δ
torchaudio/backend/soundfile_backend.py	97.01% <ø> (ø)
torchaudio/__init__.py	91.30% <100.00%> (-0.70%)	⬇️
torchaudio/backend/__init__.py	100.00% <100.00%> (ø)
torchaudio/backend/sox_backend.py	85.55% <100.00%> (ø)
torchaudio/backend/utils.py	92.00% <100.00%> (ø)

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[vincentqb]

LGTM

[mthrok]

Thanks