[Feature] Generate ann_file for flyingthing3d_subset

[MeowZheng]

Motivation

It is for generating samples filename of flyingthings3d_subset dataset

Modification

Add tools/prepare_datasets/prepare_flyingthings3d_subset.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.99%. Comparing base (579b68b) to head (c18f2fc).
⚠️ Report is 36 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
+ Coverage   68.78%   68.99%   +0.21%     
==========================================
  Files          92       92              
  Lines        5135     5135              
  Branches      727      727              
==========================================
+ Hits         3532     3543      +11     
+ Misses       1489     1478      -11     
  Partials      114      114              

Flag	Coverage Δ
unittests	68.99% <ø> (+0.21%)	⬆️

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