[Fix] fix CosineRestart eta_min

[Z-Fran]

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Motivation

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Modification

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BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
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

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.

[RangiLyu]

add some unit tests to cover this case

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@dc01545). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head 9edad80 differs from pull request most recent head 152c029. Consider uploading reports for the commit 152c029 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #639   +/-   ##
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  Coverage        ?   78.49%           
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  Files           ?      126           
  Lines           ?     9132           
  Branches        ?     1816           
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  Hits            ?     7168           
  Misses          ?     1652           
  Partials        ?      312           

Flag	Coverage Δ
unittests	78.49% <0.00%> (?)

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[HAOCHENYE]

We should make sure the added unit test will throw an error if the code has not been modified.