[Fix] add seed to distributed sampler

[fangyixiao18]

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Motivation

Modify distributed sampler and keep the manual_seed behavior same with pytorch original version
Refer to open-mmlab/mmdetection#7432 and open-mmlab/mmdetection#7440

Modification

add seed to distributed sampler and apply g.manual_seed(self.epoch + self.seed)
synchronize the seed in init function

BC-breaking (Optional)

Does the modification introduce changes that break the backward compatibility of the downstream repositories?
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

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #250 (b58024b) into dev_v0.8.0 (3dce8db) will decrease coverage by 0.16%.
The diff coverage is 35.00%.

@@              Coverage Diff               @@
##           dev_v0.8.0     #250      +/-   ##
==============================================
- Coverage       69.21%   69.05%   -0.17%     
==============================================
  Files             105      106       +1     
  Lines            3726     3745      +19     
  Branches          604      607       +3     
==============================================
+ Hits             2579     2586       +7     
- Misses           1041     1053      +12     
  Partials          106      106              

Flag	Coverage Δ
unittests	69.05% <35.00%> (-0.17%)	⬇️

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

Impacted Files	Coverage Δ
mmselfsup/datasets/builder.py	76.92% <ø> (ø)
mmselfsup/utils/dist_utils.py	31.25% <31.25%> (ø)
mmselfsup/datasets/samplers/distributed_sampler.py	16.66% <33.33%> (+0.62%)	⬆️
mmselfsup/utils/__init__.py	100.00% <100.00%> (ø)

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[YuanLiuuuuuu]

Please add UT to pass codecov

[fangyixiao18]

Please add UT to pass codecov

As this is a distributed function, it is unable to test it properly.