Add training scripts for LayoutLMv2 model
Author: harsha070
Pull request: yes

# What does this PR do?

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As discussed with @NielsRogge https://github.com/huggingface/transformers/issues/14110#issuecomment-957675691 , I added training script for LayoutLMv2 model

## Changes

Training script for LayoutLMv2

1. Using HF Trainer API
2. With HF Accelerator and without HF Trainer API
3. Corresponding documentation

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Fixes # (issue)

https://github.com/huggingface/transformers/issues/14110

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

@NielsRogge @sgugger 

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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Comments:
Awesome, thanks for adding this! Wonder if we can add it to the main examples/pytorch folder, rather than the research projects folder (cc @LysandreJik).

Also, did you run the script yourself?

Hi @harsha070 could you take into account the changes of the review?

Thanks :)

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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