

TITLE
Add PLBart

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# What does this PR do?
This PR adds PLBART.
- Paper: https://arxiv.org/abs/2103.06333
- Code and Checkpoints: https://github.com/wasiahmad/PLBART
- Authors: @wasiahmad @kaiweichang

Motivation: This encoder-decoder BART-like model allows for downstream fine-tuning on code summarization, generation, classification, etc. and is useful to the community. The fine-tuned checkpoints are also available and can be used directly.

EDIT 1:
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I am trying to make the embeddings same for the original and my implementation. Have created an issue [here](#13481) as I'm stuck with `padding_idx` outputs not matching.

### Pending Tasks
- [ ] Fix FastTokenizer
    - [ ] Tokenizer
    - ~MultiTokenizer~ 
- [x] Fix tests
    - [x] Modeling
    - [x] Tokenizer
    - ~MultiTokenizer~
- [ ] Verify behavior for all checkpoints
- [x] Update Docs
- [ ] Update Model Cards
- [x] Add remaining checkpoints
    - [x] `plbart-large`
    - [x] `plbart-base-csnet` checkpoints