[Docs] Update AVA processing data README

[congee524]

Now we only support raw frames for AVA dataset.

[codecov]

Codecov Report

Merging #801 (b6dc74d) into master (6a252b8) will decrease coverage by 0.28%.
The diff coverage is 92.30%.

❗ Current head b6dc74d differs from pull request most recent head a68089b. Consider uploading reports for the commit a68089b to get more accurate results

@@            Coverage Diff             @@
##           master     #801      +/-   ##
==========================================
- Coverage   85.19%   84.91%   -0.29%     
==========================================
  Files         130      131       +1     
  Lines        9418     9411       -7     
  Branches     1591     1602      +11     
==========================================
- Hits         8024     7991      -33     
- Misses        997     1018      +21     
- Partials      397      402       +5     

Flag	Coverage Δ
unittests	84.91% <92.30%> (-0.28%)	⬇️

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

Impacted Files	Coverage Δ
mmaction/models/__init__.py	100.00% <ø> (ø)
mmaction/models/recognizers/recognizer2d.py	90.24% <75.00%> (ø)
mmaction/models/recognizers/recognizer3d.py	96.29% <75.00%> (ø)
mmaction/models/heads/trn_head.py	93.47% <93.47%> (ø)
mmaction/models/heads/__init__.py	100.00% <100.00%> (ø)
mmaction/models/recognizers/base.py	67.66% <100.00%> (-0.72%)	⬇️
mmaction/models/losses/ohem_hinge_loss.py	75.75% <0.00%> (-24.25%)	⬇️
mmaction/models/localizers/base.py	56.86% <0.00%> (-2.40%)	⬇️
mmaction/datasets/pipelines/augmentations.py	92.75% <0.00%> (-2.11%)	⬇️
... and 26 more

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