Remove is_coco argument from test()

[glenn-jocher]

@AyushExel I'm removing the is_coco argument from test() in test.py, we'll do the determination directly in test(). This should reduce arguments and simplify things a bit.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Streamlining the is_coco parameter across test and train scripts.

📊 Key Changes

• Removed the is_coco parameter from the function signatures in test.py and train.py.
• Added automatic COCO dataset detection in test.py based on the dataset's validation file path.

🎯 Purpose & Impact

• Simplicity: Removes the need for manual specification of the is_coco flag, reducing the complexity of method calls.
• Automation: Enhances the user experience by auto-detecting if the dataset used is COCO, which could reduce configuration errors and streamline dataset handling.
• Potential Impact: Users no longer have to worry about a separate is_coco flag during testing and training, which simplifies the codebase and reduces potential bugs or misconfigurations.

[AyushExel]

@glenn-jocher awesome looks good!