[Fix] math notation in docstring

[wangruohui]

Standalone * in docstring is not correctly rendered. Previous \* not either. Try :math: notation following PyTorch.

[codecov]

Codecov Report

Merging #741 (31394c6) into master (efa1818) will decrease coverage by 0.08%.
The diff coverage is n/a.

❗ Current head 31394c6 differs from pull request most recent head 2dc6698. Consider uploading reports for the commit 2dc6698 to get more accurate results

@@            Coverage Diff             @@
##           master     #741      +/-   ##
==========================================
- Coverage   83.03%   82.95%   -0.09%     
==========================================
  Files         216      215       -1     
  Lines       12192    12121      -71     
  Branches     1964     1945      -19     
==========================================
- Hits        10124    10055      -69     
+ Misses       1764     1762       -2     
  Partials      304      304              

Flag	Coverage Δ
unittests	82.93% <ø> (-0.08%)	⬇️

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Impacted Files	Coverage Δ
mmedit/models/common/linear_module.py	100.00% <ø> (ø)
mmedit/utils/setup_env.py	95.45% <0.00%> (-4.55%)	⬇️
mmedit/models/restorers/basicvsr.py	68.88% <0.00%> (-2.83%)	⬇️
mmedit/apis/restoration_video_inference.py	17.85% <0.00%> (-2.49%)	⬇️
mmedit/datasets/builder.py	93.10% <0.00%> (-1.73%)	⬇️
mmedit/datasets/sr_folder_video_dataset.py	98.55% <0.00%> (-0.07%)	⬇️
mmedit/datasets/pipelines/augmentation.py	98.49% <0.00%> (-0.01%)	⬇️
mmedit/models/common/__init__.py	100.00% <0.00%> (ø)
mmedit/datasets/sr_reds_dataset.py	100.00% <0.00%> (ø)
mmedit/datasets/sr_vid4_dataset.py	100.00% <0.00%> (ø)
... and 7 more

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[wangruohui]

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https://mmediting--741.org.readthedocs.build/zh_CN/741/api.html?highlight=linear#mmedit.models.common.LinearModule.forward