Simple fix to Issue 158 (and potentially other issues)

[cgnorthcutt]

Fixes #158

A simple fix to clip the diagonal confident joint so that it avoids columns entirely of zero and always requires one correctly labeled example in every class.

• This is the lowest-level place where the confident joint is computed. The fix (should) trickle to all downstream use cases.
  • In your review, consider if this is the case.
• this PR reduces the warnings in our tests by half

[codecov]

Codecov Report

Merging #178 (afad588) into master (c4e8462) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #178      +/-   ##
==========================================
+ Coverage   87.25%   87.37%   +0.12%     
==========================================
  Files          12       12              
  Lines        1036     1038       +2     
  Branches      198      198              
==========================================
+ Hits          904      907       +3     
  Misses        106      106              
+ Partials       26       25       -1     

Impacted Files	Coverage Δ
cleanlab/count.py	94.96% <100.00%> (+0.07%)	⬆️
cleanlab/filter.py	91.77% <0.00%> (+0.63%)	⬆️

---

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 c4e8462...afad588. Read the comment docs.

[jwmueller]

LGTM

[anishathalye]

I rebased onto c4e8462.

[anishathalye]

Summarizing discussion offline: decided to skip warning in this situation.

LGTM.