[Refactor] Refactor ema hook

[HAOCHENYE]

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Motivation

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Modification

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BC-breaking (Optional)

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Use cases (Optional)

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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 MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (refactor_hooks_unittest@08d1a78). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head 7db93f1 differs from pull request most recent head 39fcc84. Consider uploading reports for the commit 39fcc84 to get more accurate results

Additional details and impacted files

@@                    Coverage Diff                     @@
##             refactor_hooks_unittest     #804   +/-   ##
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  Coverage                           ?   78.69%           
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  Files                              ?      128           
  Lines                              ?     9335           
  Branches                           ?     1846           
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  Hits                               ?     7346           
  Misses                             ?     1675           
  Partials                           ?      314           

Flag	Coverage Δ
unittests	78.69% <0.00%> (?)

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