Grid indices overflow bug fix

[glenn-jocher]

This PR addresses bug report #1312. In extremely rare cases, when training with mixed precision the grid index values in compute_targets() may experience overflow. This solves this problem by clamping the indices to their limits.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improvements to grid clamping in build targets and dynamic image resizing in plot images.

📊 Key Changes

• Imported math module is moved to organize imports in general.py.
• Updated grid clamping logic in the build_targets function to prevent out-of-bound grid indices.
• Modified image resizing behavior in plot_images to cap the image size for enhancing visualization and reducing memory usage.

🎯 Purpose & Impact

• 🎲 Grid Clamping: To ensure generated grid indices do not exceed model's grid dimensions, enhancing model stability.
• 🔍 Dynamic Image Resizing: To dynamically adjust the image size during plotting to optimize for visibility while keeping file sizes manageable.
• 🖼️ Visualization Improvement: Better plotting of images without degradation or excessive resizing, resulting in improved user experience when reviewing output images.

[glenn-jocher]

Also observed in YOLOv3 ultralytics/yolov3#1551