[Fix] Fix bug of CosineAnnealingParamScheduler

[fangyixiao18]

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Motivation

As T_max could be None, so we need to check its value.

Modification

Add value check of T-max in CosineAnnealingParamScheduler

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

Base: 78.41% // Head: 78.40% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (0da0bda) compared to base (e98d262).
Patch coverage: 50.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #735      +/-   ##
==========================================
- Coverage   78.41%   78.40%   -0.01%     
==========================================
  Files         127      127              
  Lines        9178     9179       +1     
  Branches     1826     1827       +1     
==========================================
  Hits         7197     7197              
  Misses       1670     1670              
- Partials      311      312       +1     

Flag	Coverage Δ
unittests	78.40% <50.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
mmengine/optim/scheduler/param_scheduler.py	87.90% <50.00%> (-0.27%)	⬇️

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