Migrate doc files to Markdown. (#24376)

* Rename index.mdx to index.md

* With saved modifs

* Address review comment

* Treat all files

* .mdx -> .md

* Remove special char

* Update utils/tests_fetcher.py

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-generation.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.
+
+-->
+
+# BertGeneration
+
+## Overview
+
+The BertGeneration model is a BERT model that can be leveraged for sequence-to-sequence tasks using
+[`EncoderDecoderModel`] as proposed in [Leveraging Pre-trained Checkpoints for Sequence Generation
+Tasks](https://arxiv.org/abs/1907.12461) by Sascha Rothe, Shashi Narayan, Aliaksei Severyn.
+
+The abstract from the paper is the following:
+
+*Unsupervised pretraining of large neural models has recently revolutionized Natural Language Processing. By
+warm-starting from the publicly released checkpoints, NLP practitioners have pushed the state-of-the-art on multiple
+benchmarks while saving significant amounts of compute time. So far the focus has been mainly on the Natural Language
+Understanding tasks. In this paper, we demonstrate the efficacy of pre-trained checkpoints for Sequence Generation. We
+developed a Transformer-based sequence-to-sequence model that is compatible with publicly available pre-trained BERT,
+GPT-2 and RoBERTa checkpoints and conducted an extensive empirical study on the utility of initializing our model, both
+encoder and decoder, with these checkpoints. Our models result in new state-of-the-art results on Machine Translation,
+Text Summarization, Sentence Splitting, and Sentence Fusion.*
+
+Usage:
+
+- The model can be used in combination with the [`EncoderDecoderModel`] to leverage two pretrained
+  BERT checkpoints for subsequent fine-tuning.
+
+```python
+>>> # leverage checkpoints for Bert2Bert model...
+>>> # use BERT's cls token as BOS token and sep token as EOS token
+>>> encoder = BertGenerationEncoder.from_pretrained("bert-large-uncased", bos_token_id=101, eos_token_id=102)
+>>> # add cross attention layers and use BERT's cls token as BOS token and sep token as EOS token
+>>> decoder = BertGenerationDecoder.from_pretrained(
+...     "bert-large-uncased", add_cross_attention=True, is_decoder=True, bos_token_id=101, eos_token_id=102
+... )
+>>> bert2bert = EncoderDecoderModel(encoder=encoder, decoder=decoder)
+
+>>> # create tokenizer...
+>>> tokenizer = BertTokenizer.from_pretrained("bert-large-uncased")
+
+>>> input_ids = tokenizer(
+...     "This is a long article to summarize", add_special_tokens=False, return_tensors="pt"
+... ).input_ids
+>>> labels = tokenizer("This is a short summary", return_tensors="pt").input_ids
+
+>>> # train...
+>>> loss = bert2bert(input_ids=input_ids, decoder_input_ids=labels, labels=labels).loss
+>>> loss.backward()
+```
+
+- Pretrained [`EncoderDecoderModel`] are also directly available in the model hub, e.g.,
+
+
+```python
+>>> # instantiate sentence fusion model
+>>> sentence_fuser = EncoderDecoderModel.from_pretrained("google/roberta2roberta_L-24_discofuse")
+>>> tokenizer = AutoTokenizer.from_pretrained("google/roberta2roberta_L-24_discofuse")
+
+>>> input_ids = tokenizer(
+...     "This is the first sentence. This is the second sentence.", add_special_tokens=False, return_tensors="pt"
+... ).input_ids
+
+>>> outputs = sentence_fuser.generate(input_ids)
+
+>>> print(tokenizer.decode(outputs[0]))
+```
+
+Tips:
+
+- [`BertGenerationEncoder`] and [`BertGenerationDecoder`] should be used in
+  combination with [`EncoderDecoder`].
+- For summarization, sentence splitting, sentence fusion and translation, no special tokens are required for the input.
+  Therefore, no EOS token should be added to the end of the input.
+
+This model was contributed by [patrickvonplaten](https://huggingface.co/patrickvonplaten). The original code can be
+found [here](https://tfhub.dev/s?module-type=text-generation&subtype=module,placeholder).
+
+## BertGenerationConfig
+
+[[autodoc]] BertGenerationConfig
+
+## BertGenerationTokenizer
+
+[[autodoc]] BertGenerationTokenizer
+    - save_vocabulary
+
+## BertGenerationEncoder
+
+[[autodoc]] BertGenerationEncoder
+    - forward
+
+## BertGenerationDecoder
+
+[[autodoc]] BertGenerationDecoder
+    - forward