[Enhance] Update digit_version function.

[mzr1996]

Motivation

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

Modification

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

BC-breaking (Optional)

No

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.

[mzr1996]

@zhouzaida

[codecov]

Codecov Report

Merging #402 (b951397) into master (a747458) will increase coverage by 0.12%.
The diff coverage is 86.95%.

@@            Coverage Diff             @@
##           master     #402      +/-   ##
==========================================
+ Coverage   76.51%   76.63%   +0.12%     
==========================================
  Files          97       97              
  Lines        5224     5234      +10     
  Branches      856      857       +1     
==========================================
+ Hits         3997     4011      +14     
+ Misses       1108     1103       -5     
- Partials      119      120       +1     

Flag	Coverage Δ
unittests	76.63% <86.95%> (+0.12%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/__init__.py	89.65% <86.95%> (+19.06%)	⬆️
mmcls/datasets/imagenet.py	25.53% <0.00%> (+1.04%)	⬆️

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[Ezra-Yu]

LGTM