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.

[mergify]

Merge Protections

Your pull request matches the following merge protections and will not be merged until they are valid.

🟢 Enforce conventional commit

Wonderful, this rule succeeded.

Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[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.