[Enhance] Add tutorial for 3D Seg task

[Wuziyi616]

Add tutorial for 3D semantic segmentation. Mainly modify from #642

We may want to specify semantic segmentation here because we will (hopefully) add instance segmentation in the future.

[codecov]

Codecov Report

Merging #663 (8ed2be8) into master (d61476a) will not change coverage.
The diff coverage is n/a.

❗ Current head 8ed2be8 differs from pull request most recent head 9a499c3. Consider uploading reports for the commit 9a499c3 to get more accurate results

@@           Coverage Diff           @@
##           master     #663   +/-   ##
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  Coverage   51.02%   51.02%           
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  Files         204      204           
  Lines       15395    15395           
  Branches     2492     2492           
=======================================
  Hits         7856     7856           
  Misses       7021     7021           
  Partials      518      518           

Flag	Coverage Δ
unittests	51.02% <ø> (ø)

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