[Feature] Support monocular 3D detection on nuScenes

[Tai-Wang]

This PR mainly supports related dataset converter and reading/evaluation API.

[codecov]

Codecov Report

Merging #392 (9ab6cf3) into master (d055876) will increase coverage by 0.58%.
The diff coverage is 56.25%.

❗ Current head 9ab6cf3 differs from pull request most recent head cd1e015. Consider uploading reports for the commit cd1e015 to get more accurate results

@@            Coverage Diff             @@
##           master     #392      +/-   ##
==========================================
+ Coverage   49.73%   50.31%   +0.58%     
==========================================
  Files         181      183       +2     
  Lines       12445    13123     +678     
  Branches     1950     2086     +136     
==========================================
+ Hits         6189     6603     +414     
- Misses       5867     6086     +219     
- Partials      389      434      +45     

Flag	Coverage Δ
unittests	50.31% <56.25%> (+0.58%)	⬆️

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Impacted Files	Coverage Δ
mmdet3d/datasets/pipelines/__init__.py	100.00% <ø> (ø)
mmdet3d/datasets/pipelines/formating.py	56.00% <0.00%> (+1.26%)	⬆️
mmdet3d/core/bbox/box_np_ops.py	26.81% <9.09%> (-0.88%)	⬇️
mmdet3d/core/evaluation/seg_eval.py	16.32% <12.50%> (-1.86%)	⬇️
mmdet3d/core/post_processing/box3d_nms.py	43.41% <20.00%> (-8.08%)	⬇️
mmdet3d/datasets/nuscenes_mono_dataset.py	32.97% <32.97%> (ø)
mmdet3d/core/bbox/transforms.py	75.00% <60.00%> (-5.96%)	⬇️
mmdet3d/core/visualizer/show_result.py	75.00% <66.66%> (-4.67%)	⬇️
mmdet3d/datasets/custom_3d_seg.py	67.14% <67.14%> (ø)
mmdet3d/datasets/pipelines/transforms_3d.py	85.13% <84.21%> (+0.79%)	⬆️
... and 11 more

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