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

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🌟 Summary

This PR modifies the letterbox function to change its stride parameter from 32 to 64.

📊 Key Changes

• Changed stride parameter in letterbox function from 32 to 64 in utils/augmentations.py.

🎯 Purpose & Impact

• Improved Image Processing: The change in stride may improve the efficiency of image resizing and padding, making operations faster.
• Better Alignment: Larger stride could help in better alignment with larger image scales, potentially enhancing model performance on larger images.
• User Noticeable Differences: Users might notice changes in model preprocessing speed and possibly minor differences in output image handling.

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