Add docstring for DistSamplerSeedHook

[MeowZheng]

No description provided.

[codecov]

Codecov Report

Merging #850 (d484881) into master (1ee5315) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #850   +/-   ##
=======================================
  Coverage   62.55%   62.55%           
=======================================
  Files         145      145           
  Lines        8755     8755           
  Branches     1591     1591           
=======================================
  Hits         5477     5477           
  Misses       3012     3012           
  Partials      266      266           

Flag	Coverage Δ
unittests	62.55% <ø> (ø)

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Impacted Files	Coverage Δ
mmcv/runner/hooks/sampler_seed.py	50.00% <ø> (ø)

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