Add mLUKE (#14640)

* implement MLukeTokenizer and LukeForMaskedLM

* update tests

* update docs

* add LukeForMaskedLM to check_repo.py

* update README

* fix test and specify the entity pad id in tokenization_(m)luke

* fix EntityPredictionHeadTransform

docs/source/model_doc/luke.rst

@@ -137,6 +137,12 @@ LukeModel
 .. autoclass:: transformers.LukeModel
     :members: forward
 
+LukeForMaskedLM
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.LukeForMaskedLM
+    :members: forward
+
 
 LukeForEntityClassification
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

docs/source/model_doc/mluke.rst

@@ -0,0 +1,66 @@
+..
+    Copyright 2021 The HuggingFace Team. All rights reserved.
+
+    Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+    the License. You may obtain a copy of the License at
+
+        http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+    an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+    specific language governing permissions and limitations under the License.
+
+mLUKE
+-----------------------------------------------------------------------------------------------------------------------
+
+Overview
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The mLUKE model was proposed in `mLUKE: The Power of Entity Representations in Multilingual Pretrained Language Models
+<https://arxiv.org/abs/2110.08151>`__ by Ryokan Ri, Ikuya Yamada, and Yoshimasa Tsuruoka. It's a multilingual extension
+of the `LUKE model <https://arxiv.org/abs/2010.01057>`__ trained on the basis of XLM-RoBERTa.
+
+It is based on XLM-RoBERTa and adds entity embeddings, which helps improve performance on various downstream tasks
+involving reasoning about entities such as named entity recognition, extractive question answering, relation
+classification, cloze-style knowledge completion.
+
+The abstract from the paper is the following:
+
+*Recent studies have shown that multilingual pretrained language models can be effectively improved with cross-lingual
+alignment information from Wikipedia entities. However, existing methods only exploit entity information in pretraining
+and do not explicitly use entities in downstream tasks. In this study, we explore the effectiveness of leveraging
+entity representations for downstream cross-lingual tasks. We train a multilingual language model with 24 languages
+with entity representations and show the model consistently outperforms word-based pretrained models in various
+cross-lingual transfer tasks. We also analyze the model and the key insight is that incorporating entity
+representations into the input allows us to extract more language-agnostic features. We also evaluate the model with a
+multilingual cloze prompt task with the mLAMA dataset. We show that entity-based prompt elicits correct factual
+knowledge more likely than using only word representations.*
+
+One can directly plug in the weights of mLUKE into a LUKE model, like so:
+
+.. code-block::
+
+    from transformers import LukeModel
+
+    model = LukeModel.from_pretrained('studio-ousia/mluke-base')
+
+Note that mLUKE has its own tokenizer, :class:`~transformers.MLukeTokenizer`. You can initialize it as follows:
+
+.. code-block::
+
+    from transformers import MLukeTokenizer
+
+    tokenizer = MLukeTokenizer.from_pretrained('studio-ousia/mluke-base')
+
+
+As mLUKE's architecture is equivalent to that of LUKE, one can refer to :doc:`LUKE's documentation page <luke>` for all
+tips, code examples and notebooks.
+
+This model was contributed by `ryo0634 <https://huggingface.co/ryo0634>`__. The original code can be found `here
+<https://github.com/studio-ousia/luke>`__.
+
+MLukeTokenizer
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. autoclass:: transformers.MLukeTokenizer
+    :members: __call__, save_vocabulary