modify default value of stride to 64 in the function letterbox

[TommeyChang]

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When the default value of stride is 32, it confronts an error when the width or height of the image is resized to 864.
I fix the bug from setting the stride to 64. Thus, I suggest to change the default value.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

This PR modifies the letterbox function within the image augmentation utilities to adapt image dimensions according to a new stride value.

📊 Key Changes

• Adjusted Stride Value: Changes the stride parameter in the letterbox function from 32 to 64.

🎯 Purpose & Impact

• Improved Performance: Increasing the stride value can enhance processing efficiency by reducing the number of calculations needed for image resizing.
• Consistency with Larger Models: This adjustment aligns the image preprocessing steps better with larger YOLO models, potentially improving compatibility and performance for larger-scale applications.
• Impact on Image Output: Users may notice changes in image padding and resizing, which should now better preserve aspect ratio and structure for models expecting larger strides. 🚀

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[github-actions]

👋 Hello @TommeyChang, thank you for submitting a YOLOv5 🚀 PR! To allow your work to be integrated as seamlessly as possible, we advise you to:

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