Ignore Seaborn plot warnings

[glenn-jocher]

Suppress empty matrix RuntimeWarning: All-NaN slice encountered when empty data (no detections) is passed to the confusion matrix.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhancements in the visualization of model metrics and label correlations.

📊 Key Changes

• Included warnings import in metrics.py to handle potential warnings.
• Wrapped heatmap generation code with a warning filter to suppress RuntimeWarnings related to empty matrices.
• Renamed the seaborn import alias from sns to sn to maintain consistency across different files.

🎯 Purpose & Impact

• The addition of warnings suppresses inconsequential runtime alerts which makes logs cleaner and user experience smoother.
• Renaming the alias to sn standardizes the codebase and avoids confusion for future development and maintenance.
• Users will benefit from a more stable tool when analyzing model performance and label distributions, without being distracted by non-critical warnings.