Quick add some explanation about the resampling in slowfast.

[SuX97]

Fixes #496

[codecov]

Codecov Report

Merging #502 (b604dba) into master (b540b6e) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #502      +/-   ##
==========================================
- Coverage   84.76%   84.71%   -0.05%     
==========================================
  Files         121      120       -1     
  Lines        8500     8407      -93     
  Branches     1394     1373      -21     
==========================================
- Hits         7205     7122      -83     
+ Misses        944      936       -8     
+ Partials      351      349       -2     

Flag	Coverage Δ
unittests	84.70% <ø> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
mmaction/models/backbones/resnet3d_slowfast.py	84.70% <ø> (ø)
mmaction/core/evaluation/accuracy.py	92.27% <0.00%> (-2.28%)	⬇️
mmaction/utils/__init__.py	100.00% <0.00%> (ø)
mmaction/models/losses/cross_entropy_loss.py	100.00% <0.00%> (ø)
mmaction/utils/precise_bn.py
mmaction/apis/train.py	15.00% <0.00%> (+1.36%)	⬆️

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[innerlee]

actual