[Enhance] Add setup multi-processing both in train and test.

[mzr1996]

Motivation

Refers to open-mmlab/mmdetection#7036

According to some tests, these setup items can avoid some problems which slow down the training speed. Test on mmcls will be added after.

Modification

Add setup multi-processing both in train and test.
Add unit tests.

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 #671 (369d386) into dev (bd397f7) will increase coverage by 0.04%.
The diff coverage is 95.83%.

@@            Coverage Diff             @@
##              dev     #671      +/-   ##
==========================================
+ Coverage   82.27%   82.31%   +0.04%     
==========================================
  Files         120      121       +1     
  Lines        7045     7068      +23     
  Branches     1228     1232       +4     
==========================================
+ Hits         5796     5818      +22     
  Misses       1075     1075              
- Partials      174      175       +1     

Flag	Coverage Δ
unittests	82.31% <95.83%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
mmcls/utils/setup_env.py	95.45% <95.45%> (ø)
mmcls/utils/__init__.py	100.00% <100.00%> (ø)

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[Ezra-Yu]

LGTM.