add new model prophetnet
# Add new model structure [ProphetNet](https://arxiv.org/abs/2001.04063).

## Description:

ProphetNet is a new pre-trained language model for sequence-to-sequence learning with a novel self-supervised objective called future n-gram prediction. ProphetNet is able to predict more future tokens with a n-stream decoder. The original implementation is Fairseq version at [github repo](https://github.com/microsoft/ProphetNet).   
xProphetNet has the same model structure, but is pretrained with wikipedia 100 languages dataset as described in [xGLUE](https://arxiv.org/abs/2004.01401). xGLUE is a benchmark for corss-lingual NLU and NLG tasks. xProphetNet is also served as a baseline model for cross-lingual generation tasks in xGLUE NTG and QG.

## Usage:

Take xGLUE NTG task as an example:  
Cross-lingual pretrained model is finetuned with English news title generation data, but inference with both English and other zero-shot language data.  
A quick usage is like: 
```
from transformers import ProphetNetTokenizer, ProphetNetForConditionalGeneration, ProphetNetConfig

model = ProphetNetForConditionalGeneration.from_pretrained('microsoft/xprophetnet-large-wiki100-cased-xglue-ntg')
tokenizer = ProphetNetTokenizer.from_pretrained('microsoft/xprophetnet-large-wiki100-cased-xglue-ntg')

EN_SENTENCE_TO_QUESTION = "Microsoft Corporation intends to officially end free support for the Windows 7 operating system after January 14, 2020, according to the official portal of the organization. From that day, users of this system will not be able to receive security updates, which could make their computers vulnerable to cyber attacks."
RU_SENTENCE_TO_QUESTION = "орпорация Microsoft намерена официально прекратить бесплатную поддержку операционной системы Windows 7 после 14 января 2020 года, сообщается на официальном портале организации . С указанного дня пользователи этой системы не смогут получать обновления безопасности, из-за чего их компьютеры могут стать уязвимыми к кибератакам."
ZH_SENTENCE_TO_QUESTION = "根据该组织的官方门户网站，微软公司打算在2020年1月14日之后正式终止对Windows 7操作系统的免费支持。从那时起，该系统的用户将无法接收安全更新，这可能会使他们的计算机容易受到网络攻击。"
inputs = tokenizer([EN_SENTENCE_TO_QUESTION, RU_SENTENCE_TO_QUESTION, ZH_SENTENCE_TO_QUESTION], padding=True, max_length=256, return_tensors='pt')

summary_ids = model.generate(inputs['input_ids'], num_beams=4, max_length=100, early_stopping=True)
print([tokenizer.decode(g) for g in summary_ids])  
```
Model will generate news titles like:
```
['[SEP] Microsoft to end Windows 7 free support after January 14, 2020[SEP][PAD][PAD][PAD][PAD]',
 '[SEP] Microsoft намерена прекратить бесплатную поддержку Windows 7 после 14 января 2020 года[SEP]',
 '[SEP]微软打算终止对Windows 7操作系统的免费支持[SEP][PAD][PAD][PAD][PAD][PAD][PAD]']
```

## Released checkpoints:

pretrained:
```
microsoft/prophetnet-large-uncased
microsoft/xprophetnet-large-wiki100-cased
```
fine-tuned:
```
microsoft/prophetnet-large-uncased-cnndm
microsoft/xprophetnet-large-wiki100-cased-xglue-ntg
microsoft/xprophetnet-large-wiki100-cased-xglue-qg
```

> @sshleifer Is there anything else needed to do in order to make ProphetNet work with your seq2seq example?
> 
> Also @mfuntowicz for pipelines

I tried examples/seq2seq/finetune.py and it works with python finetune.py --do_train and --do_predict.<|||||>I will try to complete document and unit test by this week<|||||>@patrickvonplaten Thanks for your review! I learned a lot, too.

@qiweizhen Please be free to contact me for discussion via WeChat if you have trouble understanding Patrick's comments or you want to have another person to double-check! Thanks for your great work!