Fix tf.py LoadImages() dataloader return values

[glenn-jocher]

Fix for #5446 (comment)

Additional LoadImages() dataloader 'string' return value defined in logging PR #4854.

@zldrobit be aware PR #4854 may affect your other TF export tools based on master that use representative_dataset generator.

🛠️ PR Summary

_{Made with ❤️ by Ultralytics Actions}

🌟 Summary

Upgrade of the representative dataset generator function with additional data handling.

📊 Key Changes

• Altered the tuple structure unpacked during dataset iteration to include a string element.

🎯 Purpose & Impact

• The change allows the generator to handle an additional string data point per dataset item. This could potentially enable more descriptive dataset outputs or support new features that rely on string-based data representation.
• Users may see improved dataset handling, particularly where a string identifier or descriptor is essential. However, this requires changes in dataset preparation to include the string component.

[zldrobit]

@glenn-jocher Thanks for reminding me. The only TF tool using representative_dataset is the int8 quantization of TFLite, which does not affect the Saved Model, GraphDef (pb) and tfjs export.