Document RAG again (#7377)

Do not merge before Monday

docs/source/model_summary.rst

@@ -672,6 +672,27 @@ DPR consists in three models:
 
 DPR's pipeline (not implemented yet) uses a retrieval step to find the top k contexts given a certain question, and then it calls the reader with the question and the retrieved documents to get the answer.
 
+RAG
+-----------------------------------------------------------------------------------------------------------------------
+
+.. raw:: html
+
+   <a href="https://huggingface.co/models?filter=rag">
+       <img alt="Models" src="https://img.shields.io/badge/All_model_pages-rag-blueviolet">
+   </a>
+   <a href="model_doc/rag.html">
+       <img alt="Doc" src="https://img.shields.io/badge/Model_documentation-rag-blueviolet">
+   </a>
+
+`Retrieval-Augmented Generation for Knowledge-Intensive NLP Tasks <https://arxiv.org/abs/2005.11401>`_,
+Patrick Lewis, Ethan Perez, Aleksandara Piktus, Fabio Petroni, Vladimir Karpukhin, Naman Goyal, Heinrich Küttler, Mike Lewis, Wen-tau Yih, Tim Rocktäschel, Sebastian Riedel, Douwe Kiela
+
+Retrieval-augmented generation ("RAG") models combine the powers of pretrained dense retrieval (DPR) and Seq2Seq models.
+RAG models retrieve docs, pass them to a seq2seq model, then marginalize to generate outputs.
+The retriever and seq2seq modules are initialized from pretrained models, and fine-tuned jointly, allowing both retrieval and generation to adapt to downstream tasks.
+
+The two models RAG-Token and RAG-Sequence are available for generation.
+
 More technical aspects
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