Add document descriptions for Python APIs

[wonjuleee]

Summary

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov-commenter]

Codecov Report

Base: 84.20% // Head: 84.63% // Increases project coverage by +0.43% 🎉

Coverage data is based on head (f712467) compared to base (66e33aa).
Patch has no changes to coverable lines.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #761      +/-   ##
===========================================
+ Coverage    84.20%   84.63%   +0.43%     
===========================================
  Files          157      157              
  Lines        21855    21855              
  Branches      4779     4779              
===========================================
+ Hits         18403    18497      +94     
+ Misses        2365     2260     -105     
- Partials      1087     1098      +11     

Flag	Coverage Δ
macos-10.15_Python-3.10	84.12% <ø> (ø)
macos-10.15_Python-3.7	83.80% <ø> (ø)
macos-10.15_Python-3.8	84.47% <ø> (+0.43%)	⬆️
macos-10.15_Python-3.9	84.06% <ø> (+<0.01%)	⬆️
ubuntu-20.04_Python-3.10	84.12% <ø> (ø)
ubuntu-20.04_Python-3.7	83.80% <ø> (ø)
ubuntu-20.04_Python-3.8	84.04% <ø> (ø)
ubuntu-20.04_Python-3.9	84.06% <ø> (ø)
windows-2019_Python-3.10	84.14% <ø> (ø)
windows-2019_Python-3.7	83.82% <ø> (ø)
windows-2019_Python-3.8	84.05% <ø> (ø)
windows-2019_Python-3.9	84.07% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datumaro/plugins/synthetic_data/image_generator.py	82.82% <0.00%> (+12.88%)	⬆️
datumaro/plugins/synthetic_data/utils.py	80.34% <0.00%> (+62.39%)	⬆️

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[vinnamkim]

LGTM.