Docstrings for SemanticSegmentationData

[ethanwharris]

What does this PR do?

Part of #957

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests? [not needed for typos/docs]
• Did you verify new and existing tests pass locally with your changes?
• If you made a notable change (that affects users), did you update the CHANGELOG?

PR review

• Is this pull request ready for review? (if not, please submit in draft mode)

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #1101 (9e7ede9) into master (7f45fdf) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1101      +/-   ##
==========================================
+ Coverage   88.21%   88.23%   +0.02%     
==========================================
  Files         281      281              
  Lines       12821    12822       +1     
==========================================
+ Hits        11310    11314       +4     
+ Misses       1511     1508       -3     

Flag	Coverage Δ
unittests	88.23% <100.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
flash/image/segmentation/data.py	98.07% <100.00%> (+0.07%)	⬆️
flash/image/segmentation/input.py	99.00% <100.00%> (+2.96%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7f45fdf...9e7ede9. Read the comment docs.