Update datasets.py

[glenn-jocher]

Minor updates to #3505, inplace accumulation.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improvements to label caching in the YOLOv5 data loading process.

📊 Key Changes

• Simplified progress bar updating in the cache_labels function by accumulating counts directly within the loop.
• Removed unnecessary variable reassignment for image/label verification counts (missing, found, empty, corrupted).

🎯 Purpose & Impact

• 🚀 Enhances code readability and efficiency during the label caching step.
• 🔍 Provides clearer real-time feedback to the user on the status of label scanning with a progressive summary (found, missing, empty, corrupted).
• 📈 Potentially reduces memory overhead by eliminating redundant assignments, though impact on overall performance is likely minimal.