[Fix] Fix 806

[irvingzhang0512]

Motivation

Fix #806

related comment

For ava, each video must have a fixed number(27030) of frames.
It seems OpenCV cannot do that opencv/opencv#9053 (comment)

It seems to me that the problem is that VideoCapture ignores all duplicate frames, while FFmpeg will return duplicates without issues

I would recommend that AVA dataset should not extract frames by OpenCV and we should remove related docs in tutorials.

For other dataset, there is no requirement on the number of frames for each video, so extracting frames by OpenCV is ok.

Modification

Remove docs and script that extracting ava rgb frames by OpenCV

[codecov]

Codecov Report

Merging #878 (0744127) into master (1056d29) will increase coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 0744127 differs from pull request most recent head 49b08d3. Consider uploading reports for the commit 49b08d3 to get more accurate results

@@            Coverage Diff             @@
##           master     #878      +/-   ##
==========================================
+ Coverage   83.26%   83.29%   +0.03%     
==========================================
  Files         129      129              
  Lines        9421     9421              
  Branches     1612     1612              
==========================================
+ Hits         7844     7847       +3     
+ Misses       1185     1184       -1     
+ Partials      392      390       -2     

Flag	Coverage Δ
unittests	83.29% <ø> (+0.03%)	⬆️

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Impacted Files	Coverage Δ
mmaction/datasets/pipelines/augmentations.py	93.24% <0.00%> (+0.13%)	⬆️
mmaction/core/evaluation/accuracy.py	93.18% <0.00%> (+0.90%)	⬆️

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