Update dataloaders.py

[glenn-jocher]

To disable pin_memory during training/val, i.e.

PIN_MEMORY=False python train.py 

🛠️ PR Summary

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🌟 Summary

Enhanced configuration for data loader memory pinning.

📊 Key Changes

• Introduced a new environment variable PIN_MEMORY with default value True.
• Replaced hardcoded pin_memory parameters in data loader functions with the new PIN_MEMORY variable.

🎯 Purpose & Impact

• User Customization: Allows users to set the pin_memory option via an environment variable, enhancing flexibility and control.
• Resource Management: This change can help optimize memory usage for different hardware setups, possibly improving performance.
• Ease of Use: Setting the pin_memory at one place (environment variable) rather than in multiple lines of code improves code maintainability and readability.