Add LayoutLM Model (#7064)

* first version

* finish test docs readme model/config/tokenization class

* apply make style and make quality

* fix layoutlm GitHub link

* fix conflict in index.rst and add layoutlm to pretrained_models.rst

* fix bug in test_parents_and_children_in_mappings

* reformat modeling_auto.py and tokenization_auto.py

* fix bug in test_modeling_layoutlm.py

* Update docs/source/model_doc/layoutlm.rst

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* Update docs/source/model_doc/layoutlm.rst

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* remove inh, add tokenizer fast, and update some doc

* copy and rename necessary class from modeling_bert to modeling_layoutlm

* Update src/transformers/configuration_layoutlm.py

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Update src/transformers/configuration_layoutlm.py

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Update src/transformers/configuration_layoutlm.py

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Update src/transformers/configuration_layoutlm.py

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Update src/transformers/modeling_layoutlm.py

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Update src/transformers/modeling_layoutlm.py

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* Update src/transformers/modeling_layoutlm.py

Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

* add mish to activations.py, import ACT2FN and import logging from utils

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

docs/source/index.rst

@@ -137,7 +137,10 @@ conversion utilities for the following models:
 27. `Bert For Sequence Generation <https://tfhub.dev/s?module-type=text-generation&subtype=module,placeholder>`_ (from Google) released with the paper
     `Leveraging Pre-trained Checkpoints for Sequence Generation Tasks
     <https://arxiv.org/abs/1907.12461>`_ by Sascha Rothe, Shashi Narayan, Aliaksei Severyn.
-28. `Other community models <https://huggingface.co/models>`_, contributed by the `community
+28. `LayoutLM <https://github.com/microsoft/unilm/tree/master/layoutlm>`_ (from Microsoft Research Asia) released with the paper
+    `LayoutLM: Pre-training of Text and Layout for Document Image Understanding
+    <https://arxiv.org/abs/1912.13318>`_ by Yiheng Xu, Minghao Li, Lei Cui, Shaohan Huang, Furu Wei, Ming Zhou.
+29. `Other community models <https://huggingface.co/models>`_, contributed by the `community
     <https://huggingface.co/users>`_.
 
 .. toctree::
@@ -227,6 +230,7 @@ conversion utilities for the following models:
     model_doc/funnel
     model_doc/lxmert
     model_doc/bertgeneration
+    model_doc/layoutlm
     internal/modeling_utils
     internal/tokenization_utils
     internal/pipelines_utils