Fix key error bug in iou

[cmalinmayor]

Proposed Change

Found a bug that wasn't caught by our tests because they are too small. No longer try to set IOU for non-existing edges.

Checklist

Go through these things before merge. Actions should run automatically to test them, but for information on how to run locally, see CONTRIBUTING.md.

• I have added tests that prove that my feature works in various situations or tests the bugfix (if applicable).
• I have checked that the tests pass and I maintained or improved test coverage (if applicable).
• I have written docstrings and checked that they render correctly in the documentation build.
• I have checked that mypy type checking passes.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.95%. Comparing base (7820cca) to head (2aecf79).

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #8      +/-   ##
==========================================
+ Coverage   83.90%   83.95%   +0.05%     
==========================================
  Files          12       12              
  Lines         292      293       +1     
==========================================
+ Hits          245      246       +1     
  Misses         47       47              

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