Simplify dataloader tqdm descriptions

[glenn-jocher]

@AyushExel this should help our tqdm dataloader messages fit better within a single line in our Colab notebooks (problem shown below) and also help avoid confusion about missing/empty labels, now combined into 'backgrounds'.

Signed-off-by: Glenn Jocher [email protected]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improvements to dataset scanning and labeling output messages in the Ultralytics YOLOv5 project.

📊 Key Changes

• Removed colorstr, check_dataset, check_requirements, and check_yaml imports, which were no longer needed directly in dataloaders.py.
• Refined printed output messages for dataset scanning to provide clearer statistics on image datasets (e.g., how many images were found, how many backgrounds, and the number of corrupt files).

🎯 Purpose & Impact

• 🧹 Cleanup: The removal of unnecessary imports declutters the code and may marginally improve the performance.
• 📈 Clarity & UX: The updated messages during dataset scanning give users more intuitive feedback on their datasets. This helps in troubleshooting issues related to missing or corrupt files more easily.
• 📊 Statistics: Combining 'missing' and 'empty' labels into 'backgrounds' provides a more compact and understandable summary for users to quickly assess the state of their datasets.