[Doc model summary] add MBart model summary (#6649)

docs/source/model_summary.rst

@@ -552,6 +552,31 @@ input becomes “My <x> very <y> .” and the target input becomes “<x> dog is
 
 The library provides a version of this model for conditional generation.
 
+MBart
+----------------------------------------------
+
+.. raw:: html
+
+   <a href="https://huggingface.co/models?filter=mbart">
+       <img alt="Models" src="https://img.shields.io/badge/All_model_pages-mbart-blueviolet">
+   </a>
+   <a href="model_doc/mbart.html">
+       <img alt="Doc" src="https://img.shields.io/badge/Model_documentation-mbart-blueviolet">
+   </a>
+
+`Multilingual Denoising Pre-training for Neural Machine Translation <https://arxiv.org/abs/2001.08210>`_ by Yinhan Liu, Jiatao Gu, Naman Goyal, Xian Li, Sergey Edunov
+Marjan Ghazvininejad, Mike Lewis, Luke Zettlemoyer.
+
+The model architecture and pre-training objective is same as BART, but MBart is trained on 25 languages 
+and is intended for supervised and unsupervised machine translation. MBart is one of the first methods 
+for pre-training a complete sequence-to-sequence model by denoising full texts in multiple languages,
+
+The library provides a version of this model for conditional generation.
+
+The `mbart-large-en-ro checkpoint <https://huggingface.co/facebook/mbart-large-en-ro>`_ can be used for english -> romanian translation.
+
+The `mbart-large-cc25 <https://huggingface.co/facebook/mbart-large-cc25>`_ checkpoint can be finetuned for other translation and summarization tasks, using code in ```examples/seq2seq/``` , but is not very useful without finetuning.
+
 .. _multimodal-models:
 
 Multimodal models