Add datasets for human mesh

[zengwang430521]

Rebase from new master branch.
Add the codes of datasets for human mesh estimation.
Add test codes and test data.

[codecov]

Codecov Report

Merging #159 into master will decrease coverage by 0.34%.
The diff coverage is 73.48%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
- Coverage   82.53%   82.18%   -0.35%     
==========================================
  Files          91       97       +6     
  Lines        5358     5573     +215     
  Branches      911      929      +18     
==========================================
+ Hits         4422     4580     +158     
- Misses        772      823      +51     
- Partials      164      170       +6     

Flag	Coverage Δ
#unittests	82.18% <73.48%> (-0.35%)	⬇️

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

Impacted Files	Coverage Δ
mmpose/datasets/datasets/mesh/mesh_h36m_dataset.py	24.07% <24.07%> (ø)
mmpose/datasets/datasets/mesh/mesh_base_dataset.py	82.92% <82.92%> (ø)
mmpose/datasets/datasets/mesh/mesh_adv_dataset.py	94.11% <94.11%> (ø)
mmpose/datasets/datasets/mesh/mesh_mix_dataset.py	95.45% <95.45%> (ø)
mmpose/datasets/datasets/__init__.py	100.00% <100.00%> (ø)
mmpose/datasets/datasets/mesh/__init__.py	100.00% <100.00%> (ø)
mmpose/datasets/datasets/mesh/mosh_dataset.py	100.00% <100.00%> (ø)
... and 3 more

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[innerlee]

The merge path is a little bit complex, could you simplify it?

Currently:

• human_mesh_dataset -> human_mesh
• human_mesh_pipeline -> human_mesh_dataset

and human_mesh is not visible yet.

This will make things very hard to manage.

Pls try to pr against master. e.g. if human_mesh_pipeline is relatively independent, then it can be merged into master first.
Then human_mesh_dataset can be merged into master.
and then human_mesh, etc.

The other way, namely everything merges into human_mesh, and then merge everything to master, is hard to maintain.

[innerlee]

need to rebase master

[innerlee]

need a following pr to update readme & changelog