[Enchance] Set a random seed when the user does not set a seed

[Junjun2016]

Motivation

Currently, when the seed is not set in the command line, the training process will not do anything with seed. This may cause a potential error in some cases (when different threads have different seeds).

• When the seed is not set in the args, it will randomly generate a seed and broadcast to all ranks.
• cfg.seed=seed

Related PR open-mmlab/mmdetection#6354, open-mmlab/mmdetection#6457, open-mmlab/mmdetection#6339

[codecov]

Codecov Report

Attention: Patch coverage is 26.66667% with 11 lines in your changes missing coverage. Please review.

Project coverage is 89.41%. Comparing base (d665f6b) to head (64cf90e).
Report is 301 commits behind head on master.

Files	Patch %	Lines
mmseg/apis/train.py	21.42%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1039      +/-   ##
==========================================
- Coverage   89.55%   89.41%   -0.15%     
==========================================
  Files         119      119              
  Lines        6626     6639      +13     
  Branches     1034     1037       +3     
==========================================
+ Hits         5934     5936       +2     
- Misses        488      499      +11     
  Partials      204      204              

Flag	Coverage Δ
unittests	89.41% <26.66%> (-0.15%)	⬇️

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[RockeyCoss]

Please merge the master branch into your branch, thank you.