Add LayoutLM Model
# Introduction
This pull request implements the LayoutLM model, as defined in [the paper](https://arxiv.org/abs/1912.13318):

```
LayoutLM: Pre-training of Text and Layout for Document Image Understanding 
Yiheng Xu, Minghao Li, Lei Cui, Shaohan Huang, Furu Wei, Ming Zhou, KDD 2020
```

LayoutLM is a simple but effective pre-training method of text and layout for document image understanding and information extraction tasks, such as form understanding and receipt understanding. LayoutLM archives the SOTA results on multiple datasets. 


# Typical workflow for including a model

Here an overview of the general workflow: 

- [x] Add model/configuration/tokenization classes.
- [ ] Add conversion scripts.
- [x] Add tests and a @slow integration test.
- [ ] Document your model.
- [ ] Finalize.

Let's detail what should be done at each step.

## Adding model/configuration/tokenization classes

Here is the workflow for adding model/configuration/tokenization classes:

- [x] Copy the python files from the present folder to the main folder and rename them, replacing `xxx` with your model
  name.
- [x] Edit the files to replace `XXX` (with various casing) with your model name.
- [x] Copy-paste or create a simple configuration class for your model in the `configuration_...` file.
- [x] Copy-paste or create the code for your model in the `modeling_...` files (PyTorch and TF 2.0).
- [x] Copy-paste or create a tokenizer class for your model in the `tokenization_...` file.

## Adding conversion scripts

Here is the workflow for the conversion scripts:

- [ ] Copy the conversion script (`convert_...`) from the present folder to the main folder.
- [ ] Edit this script to convert your original checkpoint weights to the current pytorch ones.

## Adding tests:

Here is the workflow for the adding tests:

- [x] Copy the python files from the `tests` sub-folder of the present folder to the `tests` subfolder of the main
  folder and rename them, replacing `xxx` with your model name.
- [x] Edit the tests files to replace `XXX` (with various casing) with your model name.
- [x] Edit the tests code as needed.

## Documenting your model:

Here is the workflow for documentation:

- [ ] Make sure all your arguments are properly documented in your configuration and tokenizer.
- [x] Most of the documentation of the models is automatically generated, you just have to make sure that
  `XXX_START_DOCSTRING` contains an introduction to the model you're adding and a link to the original
  article and that `XXX_INPUTS_DOCSTRING` contains all the inputs of your model.
- [x] Create a new page `xxx.rst` in the folder `docs/source/model_doc` and add this file in `docs/source/index.rst`.

Make sure to check you have no sphinx warnings when building the documentation locally and follow our
[documentaiton guide](https://github.com/huggingface/transformers/tree/master/docs#writing-documentation---specification).

## Final steps

You can then finish the addition step by adding imports for your classes in the common files:

- [x] Add import for all the relevant classes in `__init__.py`.
- [x] Add your configuration in `configuration_auto.py`.
- [x] Add your PyTorch and TF 2.0 model respectively in `modeling_auto.py` and `modeling_tf_auto.py`.
- [x] Add your tokenizer in `tokenization_auto.py`.
- [ ] Add a link to your conversion script in the main conversion utility (in `commands/convert.py`)
- [ ] Edit the PyTorch to TF 2.0 conversion script to add your model in the `convert_pytorch_checkpoint_to_tf2.py`
  file.
- [x] Add a mention of your model in the doc: `README.md` and the documentation itself
  in `docs/source/index.rst` and `docs/source/pretrained_models.rst`.
- [x] Upload the pretrained weights, configurations and vocabulary files.
- [ ] Create model card(s) for your models on huggingface.co. For those last two steps, check the
  [model sharing documentation](https://huggingface.co/transformers/model_sharing.html).
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7064?src=pr&el=h1) Report
> Merging [#7064](https://codecov.io/gh/huggingface/transformers/pull/7064?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/7cbf0f722d23440f3342aafc27697b50ead5996b?el=desc) will **decrease** coverage by `0.09%`.
> The diff coverage is `31.25%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/7064/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/7064?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #7064      +/-   ##
==========================================
- Coverage   80.32%   80.23%   -0.10%     
==========================================
  Files         174      177       +3     
  Lines       33446    33878     +432     
==========================================
+ Hits        26867    27181     +314     
- Misses       6579     6697     +118     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/7064?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_layoutlm.py](https://codecov.io/gh/huggingface/transformers/pull/7064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19sYXlvdXRsbS5weQ==) | `25.56% <25.56%> (ø)` | |
| [src/transformers/activations.py](https://codecov.io/gh/huggingface/transformers/pull/7064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9hY3RpdmF0aW9ucy5weQ==) | `86.36% <50.00%> (-3.64%)` | :arrow_down: |
| [src/transformers/configuration\_layoutlm.py](https://codecov.io/gh/huggingface/transformers/pull/7064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2xheW91dGxtLnB5) | `80.00% <80.00%> (ø)` | |
| [src/transformers/\_\_init\_\_.py](https://codecov.io/gh/huggingface/transformers/pull/7064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9fX2luaXRfXy5weQ==) | `99.37% <100.00%> (+0.01%)` | :arrow_up: |
| [src/transformers/configuration\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9jb25maWd1cmF0aW9uX2F1dG8ucHk=) | `96.20% <100.00%> (+0.04%)` | :arrow_up: |
| [src/transformers/modeling\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ19hdXRvLnB5) | `82.46% <100.00%> (+0.08%)` | :arrow_up: |
| [src/transformers/tokenization\_auto.py](https://codecov.io/gh/huggingface/transformers/pull/7064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fYXV0by5weQ==) | `92.30% <100.00%> (+0.12%)` | :arrow_up: |
| [src/transformers/tokenization\_layoutlm.py](https://codecov.io/gh/huggingface/transformers/pull/7064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90b2tlbml6YXRpb25fbGF5b3V0bG0ucHk=) | `100.00% <100.00%> (ø)` | |
| [src/transformers/modeling\_tf\_funnel.py](https://codecov.io/gh/huggingface/transformers/pull/7064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mdW5uZWwucHk=) | `18.53% <0.00%> (-75.51%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_flaubert.py](https://codecov.io/gh/huggingface/transformers/pull/7064/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9mbGF1YmVydC5weQ==) | `24.53% <0.00%> (-63.81%)` | :arrow_down: |
| ... and [18 more](https://codecov.io/gh/huggingface/transformers/pull/7064/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/7064?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/7064?src=pr&el=footer). Last update [7cbf0f7...4e95220](https://codecov.io/gh/huggingface/transformers/pull/7064?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Hi, I have moved and renamed the classes from modeling_bert to modeling_layoutlm and the code is ready for review and merge. @JetRunner @sgugger <|||||>@JetRunner @sgugger @LysandreJik The new suggestions have been adopted. 