[Feature] Add seed option for Sampler

[congee524]

fix #640

[codecov]

Codecov Report

Merging #642 (a3b819d) into master (1a896f2) will increase coverage by 0.03%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master     #642      +/-   ##
==========================================
+ Coverage   84.71%   84.74%   +0.03%     
==========================================
  Files         127      127              
  Lines        8967     8968       +1     
  Branches     1510     1510              
==========================================
+ Hits         7596     7600       +4     
+ Misses       1009     1007       -2     
+ Partials      362      361       -1     

Flag	Coverage Δ
unittests	84.73% <25.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
mmaction/datasets/builder.py	43.18% <0.00%> (ø)
mmaction/datasets/samplers/distributed_sampler.py	22.22% <28.57%> (-0.64%)	⬇️
mmaction/datasets/pipelines/augmentations.py	94.83% <0.00%> (+0.50%)	⬆️

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