Put structs common to backends in backend.common

[mthrok]

It is strange that sox_backend depends on custom classes defined in soudfile_backend. This PR introduces backend.common module where these custom classes are defined and have both sox_backend and soundfile_backend depend on it.

[codecov]

Codecov Report

Merging #706 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #706   +/-   ##
=======================================
  Coverage   89.28%   89.29%           
=======================================
  Files          24       25    +1     
  Lines        2342     2344    +2     
=======================================
+ Hits         2091     2093    +2     
  Misses        251      251           

Impacted Files	Coverage Δ
torchaudio/backend/__init__.py	100.00% <100.00%> (ø)
torchaudio/backend/common.py	100.00% <100.00%> (ø)
torchaudio/backend/soundfile_backend.py	96.22% <100.00%> (-0.79%)	⬇️
torchaudio/backend/sox_backend.py	85.55% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e364289...ea18ecd. Read the comment docs.

[vincentqb]

LGTM

[mthrok]

Thanks!