[Fix] Fix a warning caused by torch.div

[Dai-Wenxun]

Motivation

Fix a warning caused by torch.div. See issue #2446.

[codecov]

Codecov Report

Patch coverage: 91.66% and project coverage change: -0.02 ⚠️

Comparison is base (07259a2) 76.97% compared to head (dc2dc08) 76.95%.

❗ Current head dc2dc08 differs from pull request most recent head 50fd192. Consider uploading reports for the commit 50fd192 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2449      +/-   ##
==========================================
- Coverage   76.97%   76.95%   -0.02%     
==========================================
  Files         159      159              
  Lines       12546    12546              
  Branches     2099     2099              
==========================================
- Hits         9657     9655       -2     
- Misses       2386     2387       +1     
- Partials      503      504       +1     

Flag	Coverage Δ
unittests	76.95% <91.66%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
mmaction/models/heads/base.py	80.95% <ø> (ø)
mmaction/models/utils/blending_utils.py	90.42% <77.77%> (-1.71%)	⬇️
mmaction/__init__.py	100.00% <100.00%> (ø)
mmaction/evaluation/metrics/acc_metric.py	81.29% <100.00%> (-0.70%)	⬇️
mmaction/models/losses/cross_entropy_loss.py	65.27% <100.00%> (+0.48%)	⬆️

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