Add MP Net 2
Author: patrickvonplaten
Pull request: yes
Labels: name: PR for Model Addition

# What does this PR do?

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Copy of #8971 that had to close because of problems with git history.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

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 albert, bert, XLM: @LysandreJik
 GPT2: @LysandreJik, @patrickvonplaten
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Benchmarks: @patrickvonplaten
 Model Cards: @julien-c
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
 rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)
 Text Generation: @patrickvonplaten, @TevenLeScao
 Blenderbot, Bart, Marian, Pegasus: @patrickvonplaten
 T5: @patrickvonplaten
 Rag: @patrickvonplaten, @lhoestq
 EncoderDecoder: @patrickvonplaten
 Longformer, Reformer: @patrickvonplaten
 TransfoXL, XLNet: @TevenLeScao, @patrickvonplaten
 examples/seq2seq: @patil-suraj
 examples/bert-loses-patience: @JetRunner
 tensorflow: @jplu
 examples/token-classification: @stefan-it
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Comments:
@patrickvonplaten Never mind, just use this PR is ok. I am ok if our work can be merged into the master quickly. 

@jplu @LysandreJik @sgugger , we all gave our thumbs-up in the old PR. It's a bit unfortunate that the authorship is slightly changed here, but the PR should be read to merge. 

Squashed commit & cherry picked on the `master` branch so that the authorship is kept in df2af6d. Closing.

@StillKeepTry thanks a lot for all of your hard work on this PR! Glad to welcome MPNet in the library!

Thanks every reviewer for helping review our work. :)