Add DistSamplerSeedHook for when runner is EpochBasedRunner

[MeowZheng]

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Motivation

• Add DistSamplerSeedHook to set epoch num to dataloader when runner is EpochBasedRunner

Modification

• register DistSamplerSeedHook hook in apis/train.py

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1449 (6048333) into master (ecd2dc4) will decrease coverage by 0.03%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master    #1449      +/-   ##
==========================================
- Coverage   90.34%   90.31%   -0.04%     
==========================================
  Files         139      139              
  Lines        8300     8303       +3     
  Branches     1393     1395       +2     
==========================================
  Hits         7499     7499              
- Misses        564      567       +3     
  Partials      237      237              

Flag	Coverage Δ
unittests	90.31% <25.00%> (-0.04%)	⬇️

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

Impacted Files	Coverage Δ
mmseg/apis/train.py	20.51% <25.00%> (-0.83%)	⬇️

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