[Improvement] Speedup AVATest

[kennymckormick]

Speed up reading csv and adding ground-truth info during ava testing.
We also remove the difficult & group flags, which are not used in ava evaluation.
The evaluation results are exactly the same.

Old (7m 52s):
==> 1.55786 seconds to Reading detection results
==> 34.0824 seconds to Reading detection results
==> 52.8981 seconds to Convert groundtruth
==> 366.015 seconds to convert detections
==> 17.5152 seconds to run_evaluator

New (5m 58s):
==> 1.38209 seconds to Reading detection results
==> 28.1567 seconds to Reading detection results
==> 0.860567 seconds to Convert groundtruth
==> 310.64 seconds to convert detections
==> 17.4251 seconds to run_evaluator

Another way to speed up testing is to increase the action_thr in test_cfg, for example, increasing it from 0.00 to 0.002 can shorten the test time by 2mins, with no performance drop (exactly the same performance):
action_thr 0.002 (4m 51s):
==> 1.41658 seconds to Reading detection results
==> 20.2866 seconds to Reading detection results
==> 0.792911 seconds to Convert groundtruth
==> 254.963 seconds to convert detections
==> 13.6164 seconds to run_evaluato

[codecov]

Codecov Report

Merging #784 (d462764) into master (78eb446) will decrease coverage by 0.20%.
The diff coverage is 37.87%.

❗ Current head d462764 differs from pull request most recent head f8d3f07. Consider uploading reports for the commit f8d3f07 to get more accurate results

@@            Coverage Diff             @@
##           master     #784      +/-   ##
==========================================
- Coverage   85.37%   85.16%   -0.21%     
==========================================
  Files         130      130              
  Lines        9401     9413      +12     
  Branches     1580     1589       +9     
==========================================
- Hits         8026     8017       -9     
- Misses        970      997      +27     
+ Partials      405      399       -6     

Flag	Coverage Δ
unittests	85.15% <37.87%> (-0.22%)	⬇️

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

Impacted Files	Coverage Δ
mmaction/apis/train.py	10.18% <4.76%> (-3.46%)	⬇️
.../evaluation/ava_evaluation/per_image_evaluation.py	82.85% <83.33%> (+1.03%)	⬆️
...tion/ava_evaluation/object_detection_evaluation.py	71.34% <100.00%> (+2.72%)	⬆️
mmaction/core/evaluation/ava_utils.py	81.74% <100.00%> (+1.44%)	⬆️
mmaction/version.py	58.33% <100.00%> (ø)
mmaction/datasets/pipelines/augmentations.py	94.85% <0.00%> (+0.50%)	⬆️

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[congee524]

LGTM
And test it on LFB, the computing time reduced to 120.4 seconds