[Refactor] Move project2image-type visualization functions to a new file

[yezhen17]

No description provided.

[codecov]

Codecov Report

Merging #480 (624ce3f) into master (f7356f4) will decrease coverage by 0.09%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #480      +/-   ##
==========================================
- Coverage   50.62%   50.52%   -0.10%     
==========================================
  Files         191      192       +1     
  Lines       14450    14454       +4     
  Branches     2367     2367              
==========================================
- Hits         7315     7303      -12     
- Misses       6643     6659      +16     
  Partials      492      492              

Flag	Coverage Δ
unittests	50.52% <60.00%> (-0.10%)	⬇️

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Impacted Files	Coverage Δ
mmdet3d/core/visualizer/open3d_vis.py	0.00% <ø> (-27.95%)	⬇️
mmdet3d/core/visualizer/image_vis.py	58.62% <58.62%> (ø)
mmdet3d/core/visualizer/show_result.py	65.48% <100.00%> (+0.30%)	⬆️

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[Wuziyi616]

I also agree to split them into two files.