Polish video/io.py to remove redundant codes

[dreamerlin]

Ref: #847

[dreamerlin]

@freewind2016

[codecov]

Codecov Report

Merging #856 (c408485) into master (1ee5315) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #856      +/-   ##
==========================================
- Coverage   62.55%   62.54%   -0.02%     
==========================================
  Files         145      145              
  Lines        8755     8751       -4     
  Branches     1591     1591              
==========================================
- Hits         5477     5473       -4     
  Misses       3012     3012              
  Partials      266      266              

Flag	Coverage Δ
unittests	62.54% <50.00%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcv/video/io.py	91.19% <50.00%> (-0.22%)	⬇️
mmcv/runner/hooks/sampler_seed.py	50.00% <0.00%> (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 1ee5315...c408485. Read the comment docs.