DDP nl fix

[glenn-jocher]

Fix for #5160 (comment)

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Improved multi-GPU training support by ensuring model parameter scaling accounts for the wrapped model.

📊 Key Changes

• Modified the retrieval of the nl (number of detection layers) to use de_parallel function when the model is in Distributed Data Parallel (DDP) mode.

🎯 Purpose & Impact

• Purpose: The change ensures that when a model is being used across multiple GPUs, the detection layers count is correctly retrieved even when the model is wrapped for parallel processing.
• Impact: This improvement could lead to more accurate scaling of hyperparameters (hyp) during multi-GPU training, which enhances model performance and training stability. Users employing DDP will benefit from accurate hyperparameters adjustments irrespective of the number of GPUs used. 🚀