[Fix] Update digit_version

[Junjun2016]

Motivation

The previous digit_version has some problems, refers to open-mmlab/mmcv#1185 and open-mmlab/mmcv#1235

Modification

Copy the digit_version from mmcv and use it to parse version.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
No

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.

[Junjun2016]

@zhouzaida

[codecov]

Codecov Report

Merging #778 (e7937c6) into master (58f5dbc) will increase coverage by 0.06%.
The diff coverage is 88.00%.

❗ Current head e7937c6 differs from pull request most recent head 82f79a8. Consider uploading reports for the commit 82f79a8 to get more accurate results

@@            Coverage Diff             @@
##           master     #778      +/-   ##
==========================================
+ Coverage   85.37%   85.44%   +0.06%     
==========================================
  Files         107      107              
  Lines        5847     5859      +12     
  Branches      960      962       +2     
==========================================
+ Hits         4992     5006      +14     
+ Misses        673      670       -3     
- Partials      182      183       +1     

Flag	Coverage Δ
unittests	85.42% <84.00%> (+0.06%)	⬆️

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Impacted Files	Coverage Δ
mmseg/__init__.py	90.32% <86.95%> (+16.63%)	⬆️
mmseg/datasets/builder.py	89.47% <100.00%> (ø)

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