feat: add batch prediction for layout predictor

[SteffiVanHees]

Add batch prediction for layout predictor.

Checklist:

• Documentation has been updated, if necessary.
• Examples have been added, if necessary.
• Tests have been added, if necessary.

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[nikos-livathinos]

@SteffiVanHees thank you for your interest to support batch layout predictions.

How do you plan to use the LayoutPredictor:predict_batch() method? Do you plan to make changes in Docling that can take advantage of the batch processing?

Also, it would be nice to extend the unit tests for the LayoutPredictor and the demo_layout_predictor CLI app.

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