Rename heads detectors

[jin-s13]

1. Rename heads

BottomUpHigherResolutionHead -> AEHigherResolutionHead
BottomUpSimpleHead -> AESimpleHead
TopDownSimpleHead->TopdownHeatmapSimpleHead
TopDownMultiStageHead->TopdownHeatmapMultiStageHead
TopDownMSMUHead->TopdownHeatmapMSMUHead
MeshHMRHead->HMRMeshHead

2. Rename detectors

BottomUp -> AssociativeEmbedding

3. move mesh_heads/* and keypoint_heads/*, to heads/
4. move discriminator.py to misc/

[codecov]

Codecov Report

Merging #667 (4ba8a1e) into master (8134b29) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 4ba8a1e differs from pull request most recent head 170d941. Consider uploading reports for the commit 170d941 to get more accurate results

@@            Coverage Diff             @@
##           master     #667      +/-   ##
==========================================
+ Coverage   82.90%   82.91%   +0.01%     
==========================================
  Files         167      166       -1     
  Lines       12505    12548      +43     
  Branches     2037     2045       +8     
==========================================
+ Hits        10367    10404      +37     
- Misses       1617     1623       +6     
  Partials      521      521              

Flag	Coverage Δ
unittests	82.91% <ø> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
mmpose/models/__init__.py	100.00% <0.00%> (ø)
mmpose/models/keypoint_heads/__init__.py
mmpose/models/mesh_heads/geometric_layers.py
mmpose/models/keypoint_heads/top_down_base_head.py
mmpose/models/detectors/bottom_up.py
mmpose/models/mesh_heads/hmr_head.py
.../models/keypoint_heads/temporal_regression_head.py
mmpose/models/mesh_heads/__init__.py
...keypoint_heads/bottom_up_higher_resolution_head.py
...pose/models/keypoint_heads/top_down_simple_head.py
... and 19 more

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[jin-s13]

I did not modify the config files for now. The old configs will also work (with warnings).