Refactor ava hook

[kennymckormick]

fix #549

Migration note :

You need to use --eval mAP during AVA models testing now.

AVADataset evaluation metrics modification:
Supported metrics before: proposal, bbox
Supported metrics after: mAP (the same as bbox before), the old proposal is removed.

[innerlee]

Still WIP? Are there any configs need to be changed? Also give good warnings to help user migrate

[innerlee]

It would be great to add some migration notes in the top post, to give detailed info on what has been changed, and how to change configs/cmds to the new way. Example: open-mmlab/mmpose#382

[codecov]

Codecov Report

Merging #567 (3a4f68d) into master (b747208) will decrease coverage by 0.63%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #567      +/-   ##
==========================================
- Coverage   84.76%   84.12%   -0.64%     
==========================================
  Files         121      121              
  Lines        8579     8580       +1     
  Branches     1416     1416              
==========================================
- Hits         7272     7218      -54     
- Misses        953     1016      +63     
+ Partials      354      346       -8     

Flag	Coverage Δ
unittests	84.11% <66.66%> (-0.64%)	⬇️

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

Impacted Files	Coverage Δ
mmaction/core/evaluation/ava_utils.py	68.79% <50.00%> (-9.93%)	⬇️
mmaction/datasets/ava_dataset.py	90.00% <100.00%> (+0.05%)	⬆️
mmaction/core/evaluation/recall.py	11.76% <0.00%> (-60.30%)	⬇️

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[innerlee]

Do you need to update modelzoo, if there are related metrics affected?

[SuX97]

Just changing names looks good to me. But how do we evaluate AR@AN and Recall?

Can metrics be ['mAP', 'AR@AN', 'Recall'] since those metrics have been calculated anyway?