Add LayoutLM processor
Author: gau-nernst
Pull request: yes

# What does this PR do?

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Fixes #27826


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- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Models:

- text models: @ArthurZucker and @younesbelkada
- vision models: @amyeroberts
- speech models: @sanchit-gandhi
- graph models: @clefourrier

Library:

- flax: @sanchit-gandhi
- generate: @gante
- pipelines: @Narsil
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @muellerzr and @pacman100

Integrations:

- deepspeed: HF Trainer/Accelerate: @pacman100
- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization (bitsandbytes, autogpt): @SunMarc and @younesbelkada

Documentation: @stevhliu and @MKhalusova

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @sanchit-gandhi
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- TensorFlow: @Rocketknight1

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@ArthurZucker and @younesbelkada

Comments:
The tests are failing because when I copy LayoutLMv2Tokenizer to LayoutLMTokenizer, `bbox` input is required. To preserve backward-compatibility, we need to check if `bbox` is None, and apply the old logic in that case. I'm not familiar with the tokenizer code, so help would be appreciated 🙏.

(feel free to ping me again for another review 😉 )

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