Fix bug when using ClassBalancedDataset

[Ezra-Yu]

Motivation

Fix bug when using dataset wrapper, refer to #514

Modification

Please briefly describe what modification is made in this PR.

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 #555 (c695350) into master (2266a59) will increase coverage by 0.03%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #555      +/-   ##
==========================================
+ Coverage   79.88%   79.91%   +0.03%     
==========================================
  Files         107      107              
  Lines        6095     6100       +5     
  Branches      988      988              
==========================================
+ Hits         4869     4875       +6     
+ Misses       1094     1093       -1     
  Partials      132      132              

Flag	Coverage Δ
unittests	79.91% <88.88%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/datasets/multi_label.py	78.94% <66.66%> (+0.56%)	⬆️
mmcls/datasets/base_dataset.py	93.18% <100.00%> (+1.22%)	⬆️
mmcls/datasets/imagenet21k.py	82.43% <100.00%> (+0.74%)	⬆️

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[mzr1996]

LGTM