[Feature] Add AVA dataset and transformation

[dreamerlin]

No description provided.

[kennymckormick]

@dreamerlin There is still conflicts need to be resolved

[codecov]

Codecov Report

Merging #266 into master will increase coverage by 0.33%.
The diff coverage is 92.35%.

@@            Coverage Diff             @@
##           master     #266      +/-   ##
==========================================
+ Coverage   84.66%   84.99%   +0.33%     
==========================================
  Files          88       89       +1     
  Lines        6038     6338     +300     
  Branches      984     1033      +49     
==========================================
+ Hits         5112     5387     +275     
- Misses        739      756      +17     
- Partials      187      195       +8     

Flag	Coverage Δ
#unittests	84.99% <92.35%> (+0.33%)	⬆️

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

Impacted Files	Coverage Δ
mmaction/datasets/ava_dataset.py	89.62% <89.62%> (ø)
mmaction/datasets/pipelines/loading.py	89.18% <91.42%> (+0.12%)	⬆️
mmaction/datasets/pipelines/augmentations.py	94.85% <94.30%> (-0.16%)	⬇️
mmaction/datasets/__init__.py	100.00% <100.00%> (ø)
mmaction/datasets/pipelines/__init__.py	100.00% <100.00%> (ø)
mmaction/core/evaluation/accuracy.py	88.88% <0.00%> (-0.97%)	⬇️

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[innerlee]

looks good