Print dataset scan only if RANK in (-1, 0)

[glenn-jocher]

Resolves multiple dataset scan printouts i.e. #7336 (comment)

🛠️ PR Summary

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🌟 Summary

Improved multi-GPU support in YOLOv5 training and dataset handling.

📊 Key Changes

• Replaced list syntax with tuple syntax when checking RANK values.
• Introduced LOCAL_RANK environment variable for better multi-GPU support.
• Conditionally display cache information based on LOCAL_RANK in datasets.

🎯 Purpose & Impact

• 🧩 Enhances compatibility with distributed computing, facilitating better support for multi-GPU training environments.
• 🚀 Potentially improves training experience on multi-GPU setups by preventing unnecessary output on all ranks except the master.
• 🖥️ Ensures that only the master process displays dataset scanning progress, reducing clutter and confusion in the console output when using multiple GPUs.

[glenn-jocher]

It works!!