On-demand pycocotools pip install

[glenn-jocher]

Comments pycocotools from requirements.txt and instead scans and installs the package on demand when needed in test.py.

Possible fix for #3543

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Updates to dependency management and conditional requirement checks in YOLOv5's various scripts.

📊 Key Changes

• 🧹 Removed pycocotools from the excluded list in check_requirements in detect.py, hubconf.py, test.py, and train.py.
• 🚫 Commented out pycocotools in requirements.txt, changing its installation from mandatory to optional.
• ✅ Added a dynamic requirement check for pycocotools within the test.py script before using it.

🎯 Purpose & Impact

• Simpler Installation: The removal of pycocotools from the default requirements reduces installation complexity for users who do not need COCO mAP functionality, making it lighter and potentially avoiding compatibility issues.
• Conditional Requirement: By checking for pycocotools only where it's necessary (test.py), the impact is that only users performing COCO evaluation will need to install this dependency.
• User Flexibility: This approach provides more flexibility for users, only prompting for installation of additional packages when they are needed for specific functionality.