fix: transformers warnings

[PeterStaar-IBM]

fix: avoid deprecated torch_dtype warnings on transformers 5.x

Description

transformers 5.x renamed the from_pretrained keyword torch_dtype to dtype.
Both Transformers-backed inference engines still passed torch_dtype=torch_dtype
unconditionally, including when the dtype resolved to None.

That is not a no-op on transformers 5.x: the None value is forwarded into the
config kwargs and reaches the deprecated PretrainedConfig.torch_dtype setter,
which emits a deprecation warning on every model load. Users converting documents
with the layout / image-classification stages saw this warning repeatedly in
otherwise clean output.

This PR:

• Only passes a dtype kwarg when a dtype was actually resolved (torch_dtype is not None),
  so the default path no longer touches the deprecated setter at all.
• Picks the correct keyword name at runtime based on the installed transformers
  major version (dtype for >= 5, torch_dtype otherwise), keeping both
  transformers 4.x and 5.x working from the same code path.
• Adds a comment documenting a second, unrelated source of noise: RT-DETRv2 on
  transformers 5.x validates encoder spatial shapes with a tensor condition, which
  causes a torch.compile graph break ("Graph break from Tensor.item()") once per
  compiled region. It is noisy but harmless, and can be silenced with
  TRANSFORMERS_DISABLE_TORCH_CHECK=1 — safe for our fixed-resolution exports.

Files touched:

• docling/models/inference_engines/image_classification/transformers_engine.py
• docling/models/inference_engines/object_detection/transformers_engine.py

No behavior change for users on transformers 4.x, and no change to the resolved
dtype in any case — only how (and whether) it is handed to from_pretrained.

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ngines/image_classification/transformers_engine.py	60.00%	2 Missing ⚠️
...ce_engines/object_detection/transformers_engine.py	60.00%	2 Missing ⚠️

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[dolfim-ibm]

lgtm