add freihand dataset

[liuxin9608]

Add freihand dataset

[codecov]

Codecov Report

Merging #125 into master will increase coverage by 0.99%.
The diff coverage is 91.30%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   81.19%   82.19%   +0.99%     
==========================================
  Files          83       84       +1     
  Lines        4760     4870     +110     
  Branches      840      857      +17     
==========================================
+ Hits         3865     4003     +138     
+ Misses        753      716      -37     
- Partials      142      151       +9     

Flag	Coverage Δ
#unittests	82.19% <91.30%> (+0.99%)	⬆️

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

Impacted Files	Coverage Δ
mmpose/datasets/datasets/__init__.py	100.00% <ø> (ø)
mmpose/apis/inference.py	77.16% <50.00%> (ø)
...sets/datasets/top_down/topdown_freihand_dataset.py	91.81% <91.81%> (ø)
mmpose/core/evaluation/top_down_eval.py	94.82% <100.00%> (-0.05%)	⬇️
mmpose/datasets/datasets/top_down/__init__.py	100.00% <100.00%> (ø)
mmpose/datasets/pipelines/top_down_transform.py	78.18% <0.00%> (-2.43%)	⬇️
...ts/datasets/top_down/topdown_onehand10k_dataset.py	94.54% <0.00%> (+37.27%)	⬆️

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[innerlee]

looks good