modify default value of stride to 64 in the function letterbox

[TommeyChang]

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.

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🛠️ PR Summary

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🌟 Summary

Small update to the image preprocessing in YOLOv5 to allow more flexible stride options.

📊 Key Changes

• Modified the letterbox function's stride parameter from a default of 32 to 64.

🎯 Purpose & Impact

• Improved Flexibility: By increasing the default stride value, images can be resized and padded with more flexible stride options.
• Potential Performance Impact: This may affect model inference times or performance based on how images are processed, possibly improving efficiency for certain applications.
• User Convenience: Users may experience improved outcomes in training and inference without needing to manually adjust stride settings.

Overall, this change aims to optimize image preprocessing, benefiting both developers and end-users by making the model slightly more adaptable and efficient. 📈✨

[github-actions]

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1 out of 2 committers have signed the CLA.
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❌ @TommeyChang
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