[Enhance] Improve accuracy calculation performance.

[mzr1996]

Motivation

The original accuracy_numpy needs twice sort which costs large memory and time if the number of classes is large.

Modification

1. Use better methods to improve accuracy_numpy's performance.
2. Fix a bug in accuracy calculation.

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 #592 (8638a7d) into master (58ab028) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #592      +/-   ##
==========================================
+ Coverage   80.01%   80.07%   +0.06%     
==========================================
  Files         109      111       +2     
  Lines        6419     6450      +31     
  Branches     1102     1108       +6     
==========================================
+ Hits         5136     5165      +29     
- Misses       1146     1149       +3     
+ Partials      137      136       -1     

Flag	Coverage Δ
unittests	80.07% <100.00%> (+0.06%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/models/losses/accuracy.py	91.66% <100.00%> (+3.43%)	⬆️
mmcls/core/hook/__init__.py	100.00% <0.00%> (ø)
mmcls/core/hook/class_num_check_hook.py	84.00% <0.00%> (ø)

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[0x4f5da2]

LGTM