Update license line in Python files

[UltralyticsAssistant]

This pull request updates or adds the license line to the beginning of each Python file.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Updated all references from "YOLOv3 by Ultralytics" to "Ultralytics YOLOv3" for consistency.

📊 Key Changes

• Header Update: Changed the header comment across various files to "Ultralytics YOLOv3 🚀, AGPL-3.0 license".
• Template Updates: Amendments in issue templates and workflow files to reflect the updated header.
• Code Files: Adjusted the header comments in multiple Python scripts including training, validation, and utility files.

🎯 Purpose & Impact

• Consistency: Ensures a standardized naming convention across the codebase.
• Clarity: Makes it explicitly clear that the YOLOv3 project belongs to Ultralytics.
• Maintenance: Simplifies future documentation and reference consistency, aiding both developers and users in recognizing the source and licensing.