Add PLBart
# 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
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>The pre-trained model conversion is working correctly. Have verified with encoder/decoder outputs and embedding outputs for encoder. 
The states match except for pad tokens, for which the issue is mentioned in #13481.
The tokenizer for the pre-trained checkpoint contains 50005 tokens, with the special ones:
```
0 <s>
1 <pad>
2 </s>
3 <unk>
50001 [java]
50002 [python]
50003 [en_XX]
50004 <mask>
```<|||||>@gchhablani - can I help you somehow to move forward with this PR? <|||||>Any update on this PR? @gchhablani <|||||>@patil-suraj
Updates:
- Rebased the branch to master.
- Updated the documentation. Should I move the generation example and other model/tokenizer docs from the model/tokenization files to the `mdx`?
- Removed all the extra changes due to merging issues.

Regarding the tokenizes, I am not sure if they can be moved to a single tokenizer considering [this comment](https://github.com/huggingface/transformers/pull/13269#discussion_r790167690). Please let me know if there's a way to do that.<|||||>@patil-suraj @gchhablani - let me know when the PR is ready for a final review :-)<|||||>@patrickvonplaten Would be awesome if you could take a final look now :) <|||||>PR is good for merge to me! Failing test is a flaky one from the Hub.<|||||>Thanks a lot @gchhablani for all your work on this! Great job!<|||||>Hi, @gchhablani @patil-suraj @sgugger 

Looks like `README.md` (and therefore `doc/source/index.mdx`) is not updated for this new added model.
Ideally, it is usually updated when a new model is added, right?
 <|||||>Thanks for flagging @ydshieh , we indeed have forgotten to ask for it during our reviews! @gchhablani would you like to add it in a follow-up PR?<|||||>@ydshieh Thanks a lot for informing! On it.

@sgugger Yes, I'll quickly fix it.