Fix for labels_map argument

[rwood-97]

Summary

This was an oversight in #489 .
When getting the label indices from labels i was using the list of unique labels, but since the most recent changes the ordering of this list no longer reflects the ordering of labels in the labels map.

Now instead, we create an inverse of the labels map and use this to find the label index.

Checklist before assigning a reviewer (update as needed)

• Self-review code
• Ensure submission passes current tests
• Add tests
• [] Update relevant docs

Reviewer checklist

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• Everything looks ok?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.79%. Comparing base (7de87ef) to head (5c9fc6f).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #490      +/-   ##
==========================================
+ Coverage   65.74%   65.79%   +0.05%     
==========================================
  Files          46       46              
  Lines        7467     7478      +11     
==========================================
+ Hits         4909     4920      +11     
  Misses       2558     2558              

Flag	Coverage Δ
unittests	65.79% <100.00%> (+0.05%)	⬆️

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