Convert model files from rst to mdx (#14865)

* First pass

* Apply suggestions from code review

* Apply suggestions from code review

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

docs/source/model_doc/dpr.mdx

@@ -0,0 +1,98 @@
+<!--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
+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.
+-->
+
+# DPR
+
+## Overview
+
+Dense Passage Retrieval (DPR) is a set of tools and models for state-of-the-art open-domain Q&A research. It was
+introduced in [Dense Passage Retrieval for Open-Domain Question Answering](https://arxiv.org/abs/2004.04906) by
+Vladimir Karpukhin, Barlas Oğuz, Sewon Min, Patrick Lewis, Ledell Wu, Sergey Edunov, Danqi Chen, Wen-tau Yih.
+
+The abstract from the paper is the following:
+
+*Open-domain question answering relies on efficient passage retrieval to select candidate contexts, where traditional
+sparse vector space models, such as TF-IDF or BM25, are the de facto method. In this work, we show that retrieval can
+be practically implemented using dense representations alone, where embeddings are learned from a small number of
+questions and passages by a simple dual-encoder framework. When evaluated on a wide range of open-domain QA datasets,
+our dense retriever outperforms a strong Lucene-BM25 system largely by 9%-19% absolute in terms of top-20 passage
+retrieval accuracy, and helps our end-to-end QA system establish new state-of-the-art on multiple open-domain QA
+benchmarks.*
+
+This model was contributed by [lhoestq](https://huggingface.co/lhoestq). The original code can be found [here](https://github.com/facebookresearch/DPR).
+
+
+## DPRConfig
+
+[[autodoc]] DPRConfig
+
+## DPRContextEncoderTokenizer
+
+[[autodoc]] DPRContextEncoderTokenizer
+
+## DPRContextEncoderTokenizerFast
+
+[[autodoc]] DPRContextEncoderTokenizerFast
+
+## DPRQuestionEncoderTokenizer
+
+[[autodoc]] DPRQuestionEncoderTokenizer
+
+## DPRQuestionEncoderTokenizerFast
+
+[[autodoc]] DPRQuestionEncoderTokenizerFast
+
+## DPRReaderTokenizer
+
+[[autodoc]] DPRReaderTokenizer
+
+## DPRReaderTokenizerFast
+
+[[autodoc]] DPRReaderTokenizerFast
+
+## DPR specific outputs
+
+[[autodoc]] models.dpr.modeling_dpr.DPRContextEncoderOutput
+
+[[autodoc]] models.dpr.modeling_dpr.DPRQuestionEncoderOutput
+
+[[autodoc]] models.dpr.modeling_dpr.DPRReaderOutput
+
+## DPRContextEncoder
+
+[[autodoc]] DPRContextEncoder
+    - forward
+
+## DPRQuestionEncoder
+
+[[autodoc]] DPRQuestionEncoder
+    - forward
+
+## DPRReader
+
+[[autodoc]] DPRReader
+    - forward
+
+## TFDPRContextEncoder
+
+[[autodoc]] TFDPRContextEncoder
+    - call
+
+## TFDPRQuestionEncoder
+
+[[autodoc]] TFDPRQuestionEncoder
+    - call
+
+## TFDPRReader
+
+[[autodoc]] TFDPRReader
+    - call