Adding support for LayoutLMvX variants for `object-detection`.
Author: Narsil
Pull request: yes

# What does this PR do?

Adding support for `layoutlm` to `object-detection`.

LayoutLMv{2,3} can be used for `object-detection`.

However the classes are `ForTokenClassification` which not all classes can support
a vision + OCR type of inference. (the model needs `bbox` object even if we splitted
out the OCR).

The current implementation changes `object-detection` to `multimodal` since
now the models require `tokenizer` for the layoutlm variants. (This does not 
affect existing working pipelines).

Then it uses reflection at runtime to see if the model is using a `tokenizer`.
This is not a great way to go about it but was the "simpler" change I could think
of.
As long as we don't have support for other model architecture, I'm hesitant to
make "cleaner" modifications, since I don't know if other architectures will
support the same invariants or not.

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Fixes # (issue)


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