add mpii demo

[jin-s13]

add demo for mpii #210

[codecov]

Codecov Report

Merging #216 into master will decrease coverage by 0.08%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #216      +/-   ##
==========================================
- Coverage   82.90%   82.82%   -0.09%     
==========================================
  Files         106      106              
  Lines        6211     6217       +6     
  Branches     1001     1003       +2     
==========================================
  Hits         5149     5149              
- Misses        874      878       +4     
- Partials      188      190       +2     

Flag	Coverage Δ
#unittests	82.82% <0.00%> (-0.09%)	⬇️

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

Impacted Files	Coverage Δ
mmpose/apis/inference.py	68.70% <0.00%> (-2.93%)	⬇️
...datasets/datasets/top_down/topdown_mpii_dataset.py	42.71% <ø> (ø)

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