Improve LayoutLM (#9476)

* Add LayoutLMForSequenceClassification and integration tests

Improve docs

Add LayoutLM notebook to list of community notebooks

* Make style & quality

* Address comments by @sgugger, @patrickvonplaten and @LysandreJik

* Fix rebase with master

* Reformat in one line

* Improve code examples as requested by @patrickvonplaten

Co-authored-by: Lysandre <lysandre.debut@reseau.eseo.fr>
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>

docs/source/model_doc/layoutlm.rst

@@ -13,32 +13,72 @@
 LayoutLM
 -----------------------------------------------------------------------------------------------------------------------
 
+.. _Overview:
+
 Overview
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The LayoutLM model was proposed in 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, and
 Ming Zhou. It's a simple but effective pretraining method of text and layout for document image understanding and
-information extraction tasks, such as form understanding and receipt understanding.
+information extraction tasks, such as form understanding and receipt understanding. It obtains state-of-the-art results
+on several downstream tasks:
+
+- form understanding: the `FUNSD <https://guillaumejaume.github.io/FUNSD/>`__ dataset (a collection of 199 annotated
+  forms comprising more than 30,000 words).
+- receipt understanding: the `SROIE <https://rrc.cvc.uab.es/?ch=13>`__ dataset (a collection of 626 receipts for
+  training and 347 receipts for testing).
+- document image classification: the `RVL-CDIP <https://www.cs.cmu.edu/~aharley/rvl-cdip/>`__ dataset (a collection of
+  400,000 images belonging to one of 16 classes).
 
 The abstract from the paper is the following:
 
 *Pre-training techniques have been verified successfully in a variety of NLP tasks in recent years. Despite the
 widespread use of pretraining models for NLP applications, they almost exclusively focus on text-level manipulation,
 while neglecting layout and style information that is vital for document image understanding. In this paper, we propose
-the \textbf{LayoutLM} to jointly model interactions between text and layout information across scanned document images,
-which is beneficial for a great number of real-world document image understanding tasks such as information extraction
-from scanned documents. Furthermore, we also leverage image features to incorporate words' visual information into
-LayoutLM. To the best of our knowledge, this is the first time that text and layout are jointly learned in a single
-framework for document-level pretraining. It achieves new state-of-the-art results in several downstream tasks,
-including form understanding (from 70.72 to 79.27), receipt understanding (from 94.02 to 95.24) and document image
-classification (from 93.07 to 94.42).*
+the LayoutLM to jointly model interactions between text and layout information across scanned document images, which is
+beneficial for a great number of real-world document image understanding tasks such as information extraction from
+scanned documents. Furthermore, we also leverage image features to incorporate words' visual information into LayoutLM.
+To the best of our knowledge, this is the first time that text and layout are jointly learned in a single framework for
+document-level pretraining. It achieves new state-of-the-art results in several downstream tasks, including form
+understanding (from 70.72 to 79.27), receipt understanding (from 94.02 to 95.24) and document image classification
+(from 93.07 to 94.42).*
 
 Tips:
 
-- LayoutLM has an extra input called :obj:`bbox`, which is the bounding boxes of the input tokens.
-- The :obj:`bbox` requires the data that on 0-1000 scale, which means you should normalize the bounding box before
-  passing them into model.
+- In addition to `input_ids`, :meth:`~transformer.LayoutLMModel.forward` also expects the input :obj:`bbox`, which are
+  the bounding boxes (i.e. 2D-positions) of the input tokens. These can be obtained using an external OCR engine such
+  as Google's `Tesseract <https://github.com/tesseract-ocr/tesseract>`__ (there's a `Python wrapper
+  <https://pypi.org/project/pytesseract/>`__ available). Each bounding box should be in (x0, y0, x1, y1) format, where
+  (x0, y0) corresponds to the position of the upper left corner in the bounding box, and (x1, y1) represents the
+  position of the lower right corner. Note that one first needs to normalize the bounding boxes to be on a 0-1000
+  scale. To normalize, you can use the following function:
+
+.. code-block::
+
+   def normalize_bbox(bbox, width, height):
+        return [
+            int(1000 * (bbox[0] / width)),
+            int(1000 * (bbox[1] / height)),
+            int(1000 * (bbox[2] / width)),
+            int(1000 * (bbox[3] / height)),
+        ]
+
+Here, :obj:`width` and :obj:`height` correspond to the width and height of the original document in which the token
+occurs. Those can be obtained using the Python Image Library (PIL) library for example, as follows:
+
+.. code-block::
+
+   from PIL import Image
+
+   image = Image.open("name_of_your_document - can be a png file, pdf, etc.")
+
+   width, height = image.size
+
+- For a demo which shows how to fine-tune :class:`LayoutLMForTokenClassification` on the `FUNSD dataset
+  <https://guillaumejaume.github.io/FUNSD/>`__ (a collection of annotated forms), see `this notebook
+  <https://github.com/NielsRogge/Transformers-Tutorials/blob/master/LayoutLM/Fine_tuning_LayoutLMForTokenClassification_on_FUNSD.ipynb>`__.
+  It includes an inference part, which shows how to use Google's Tesseract on a new document.
 
 The original code can be found `here <https://github.com/microsoft/unilm/tree/master/layoutlm>`_.
 
@@ -78,6 +118,13 @@ LayoutLMForMaskedLM
     :members:
 
 
+LayoutLMForSequenceClassification
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.LayoutLMForSequenceClassification
+    :members:
+
+
 LayoutLMForTokenClassification
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~