[Enhance] speed up training

[fangyixiao18]

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Motivation

To speed up training

Modification

Refer to open-mmlab/mmdetection#6974
modify default_runtime.py, add opencv_num_threads = 0 and mp_start_method = 'fork'
modify tools/train.py

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.

[Jiahao000]

Does setup_multi_processes also work for tools/test.py?

[fangyixiao18]

Does setup_multi_processes also work for tools/test.py?

Theoretically, it works. I will revise the code later according to the newest pr of mmdet.

[fangyixiao18]

The update refers to open-mmlab/mmdetection#7036

[codecov]

Codecov Report

Merging #181 (6a6dda0) into master (cc19b34) will increase coverage by 3.84%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
+ Coverage   59.97%   63.82%   +3.84%     
==========================================
  Files          94       95       +1     
  Lines        3388     3411      +23     
  Branches      542      546       +4     
==========================================
+ Hits         2032     2177     +145     
+ Misses       1271     1141     -130     
- Partials       85       93       +8     

Flag	Coverage Δ
unittests	63.82% <95.65%> (+3.84%)	⬆️

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

Impacted Files	Coverage Δ
mmselfsup/utils/setup_env.py	95.45% <95.45%> (ø)
mmselfsup/utils/__init__.py	100.00% <100.00%> (ø)
mmselfsup/datasets/builder.py	72.41% <0.00%> (+12.06%)	⬆️
mmselfsup/utils/clustering.py	86.45% <0.00%> (+74.83%)	⬆️

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