Plot labels.png histogram colors

[glenn-jocher]

Solution for #1879 (comment)

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Enhanced the label histogram color-coding in the label plotting utility.

📊 Key Changes

• The histogram ax[0] in plot_labels will now color code each class bin.
• A list comprehension is added to set the color for each patch in the histogram based on class indices.

🎯 Purpose & Impact

• Purpose: To provide a visual distinction between different classes in the histogram, making it more readable and informative.
• Impact: Users will benefit from a clearer visual representation of their data classes, improving their ability to interpret the distribution of classes within datasets. 🎨📈

[glenn-jocher]

Updated labels.png with histogram bars now colored (for VOC):