[Fix] Fix potential unexcepted behaviors if metric_options is not specified in multi-label evaluation.

[dreamer121121]

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Motivation

muti-label test bug, default metric-options is {}.

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the backward compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

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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.

[CLAassistant]

All committers have signed the CLA.

[mzr1996]

LGTM

[mzr1996]

@dreamer121121 Thanks for your contribution. Please sign the CLA and then we can merge your code.

[codecov]

Codecov Report

Merging #647 (7cbc987) into dev (a451d63) will not change coverage.
The diff coverage is 0.00%.

❗ Current head 7cbc987 differs from pull request most recent head f55ab6f. Consider uploading reports for the commit f55ab6f to get more accurate results

@@           Coverage Diff           @@
##              dev     #647   +/-   ##
=======================================
  Coverage   82.27%   82.27%           
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  Files         120      120           
  Lines        7045     7045           
  Branches     1228     1228           
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  Hits         5796     5796           
  Misses       1075     1075           
  Partials      174      174           

Flag	Coverage Δ
unittests	82.27% <0.00%> (ø)

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

Impacted Files	Coverage Δ
mmcls/datasets/multi_label.py	75.00% <0.00%> (ø)

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[mzr1996]

LGTM