Migrate doc files to Markdown. (#24376)

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

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

docs/source/en/model_doc/bert-japanese.md

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+<!--Copyright 2020 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
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+
+⚠️ Note that this file is in Markdown but contain specific syntax for our doc-builder (similar to MDX) that may not be
+rendered properly in your Markdown viewer.
+
+-->
+
+# BertJapanese
+
+## Overview
+
+The BERT models trained on Japanese text.
+
+There are models with two different tokenization methods:
+
+- Tokenize with MeCab and WordPiece. This requires some extra dependencies, [fugashi](https://github.com/polm/fugashi) which is a wrapper around [MeCab](https://taku910.github.io/mecab/).
+- Tokenize into characters.
+
+To use *MecabTokenizer*, you should `pip install transformers["ja"]` (or `pip install -e .["ja"]` if you install
+from source) to install dependencies.
+
+See [details on cl-tohoku repository](https://github.com/cl-tohoku/bert-japanese).
+
+Example of using a model with MeCab and WordPiece tokenization:
+
+```python
+>>> import torch
+>>> from transformers import AutoModel, AutoTokenizer
+
+>>> bertjapanese = AutoModel.from_pretrained("cl-tohoku/bert-base-japanese")
+>>> tokenizer = AutoTokenizer.from_pretrained("cl-tohoku/bert-base-japanese")
+
+>>> ## Input Japanese Text
+>>> line = "吾輩は猫である。"
+
+>>> inputs = tokenizer(line, return_tensors="pt")
+
+>>> print(tokenizer.decode(inputs["input_ids"][0]))
+[CLS] 吾輩 は 猫 で ある 。 [SEP]
+
+>>> outputs = bertjapanese(**inputs)
+```
+
+Example of using a model with Character tokenization:
+
+```python
+>>> bertjapanese = AutoModel.from_pretrained("cl-tohoku/bert-base-japanese-char")
+>>> tokenizer = AutoTokenizer.from_pretrained("cl-tohoku/bert-base-japanese-char")
+
+>>> ## Input Japanese Text
+>>> line = "吾輩は猫である。"
+
+>>> inputs = tokenizer(line, return_tensors="pt")
+
+>>> print(tokenizer.decode(inputs["input_ids"][0]))
+[CLS] 吾 輩 は 猫 で あ る 。 [SEP]
+
+>>> outputs = bertjapanese(**inputs)
+```
+
+Tips:
+
+- This implementation is the same as BERT, except for tokenization method. Refer to the [documentation of BERT](bert) for more usage examples.
+
+This model was contributed by [cl-tohoku](https://huggingface.co/cl-tohoku).
+
+## BertJapaneseTokenizer
+
+[[autodoc]] BertJapaneseTokenizer