[Refactor] Refactor checkpointhook unittest

[HAOCHENYE]

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Motivation

If the metric name contains / character, such as 'top10/acc', CheckpointHook will first create the best_top10 directory and save the best checkpoint in it, which is not our expected:

best_top10
└── acc_epoch_5.pth

This PR will replace all / with _ to prevent from creating an unnecessary directory.

best_top10_acc_epoch_5.pth

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 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)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

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 (main@fd84c21). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head 75bc93c differs from pull request most recent head cdc86ef. Consider uploading reports for the commit cdc86ef to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #789   +/-   ##
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  Coverage        ?   76.69%           
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  Files           ?      139           
  Lines           ?    11035           
  Branches        ?     2207           
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  Hits            ?     8463           
  Misses          ?     2209           
  Partials        ?      363           

Flag	Coverage Δ
unittests	76.69% <0.00%> (?)

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