Add compatibility with GIF images

[FTD-YI]

Some images have the extension PNG but are actually GIF， the dataset file needs to handle this case.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced image loading to support GIFs in YOLOv5.

📊 Key Changes

• Modified image loading logic to handle GIF files using OpenCV.
• Incorporated a conditional check to read a frame from GIF if standard image loading fails.
• Ensured proper release of GIF resources after reading.

🎯 Purpose & Impact

• Purpose: Allows YOLOv5 to process GIF images directly, expanding the range of compatible image formats.
• Impact: Users can now use GIFs in their datasets without pre-converting them to static image formats, simplifying workflows and enabling new use cases. 🔄🖼️

[github-actions]

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[glenn-jocher]

@FTD-YI it's not clear that GIFs should be included in training data, as there is ambiguity as to which frame of the GIF the labels.txt file would apply.

After investigating a bit I've added gif to the included suffix list for videos in #6410, allowing detect.py full gif inference capability (handling gifs as videos, running inference on every frame).

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions YOLOv5 🚀 and Vision AI ⭐.