Update val.py --speed and --study usages

[glenn-jocher]

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhancements to the val.py script facilitating easier speed benchmarks and studies with varying YOLOv5 models.

📊 Key Changes

• Added a comment line in the speed task section for better usage clarity.
• Updated the comment line in the study task section to include lighter models.

🎯 Purpose & Impact

• 🛠 Usability Improvement: The added comments serve as quick-use examples directly in the code, making it easier for users to understand how to execute speed benchmarks and studies.
• 📈 Broader Model Support: By including yolov5n.pt (a lighter model) in the example commands, users are encouraged to test across a wider range of models for comprehensive performance insights.
• ✅ Potential User Impact: Both novice and advanced users benefit from clear examples within the code, reducing the learning curve and simplifying the testing process across different model sizes.