Bug fix: ensure SS datasets always return label array with correct dtype

[AdeelH]

Overview

This PR fixes a bug, reported in #1950, where SS datasets would return y without converting it to the right integer dtype first.

Checklist

• Added needs-backport label if PR is bug fix that applies to previous minor release
• Ran scripts/format_code and committed any changes
• Documentation updated if needed
• PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

• See new unit tests.

[codecov]

Codecov Report

Merging #1954 (4adf90e) into master (89e8ce7) will increase coverage by 0.02%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1954      +/-   ##
==========================================
+ Coverage   82.46%   82.48%   +0.02%     
==========================================
  Files         190      190              
  Lines        9336     9337       +1     
==========================================
+ Hits         7699     7702       +3     
+ Misses       1637     1635       -2     

Files	Coverage Δ
.../rastervision/pytorch_learner/dataset/transform.py	95.96% <100.00%> (+0.84%)	⬆️

... and 1 file with indirect coverage changes