Special Language Token for PLBART needs to be updated
### System Info

The `FAIRSEQ_LANGUAGE_CODES` in PLBartTokenizer [here](https://github.com/huggingface/transformers/blob/main/src/transformers/models/plbart/tokenization_plbart.py#L90) need to be as follows.

```
FAIRSEQ_LANGUAGE_CODES = {
    "base": ["__java__", "__python__", "__en_XX__"],
    "multi": ["__java__", "__python__", "__en_XX__", "__javascript__", "__php__", "__ruby__", "__go__"],
}
```

The current PLBartTokenizer treats `java` as a special token, and thus it removes the token when decoding is performed. An example is given below.

```
code = "public void METHOD_1 ( TYPE_1 VAR_1 ) throws java.lang.Exception { super . METHOD_1 ( VAR_1 ) ; METHOD_2 ( VAR_1 ) ; }"
tokenizer = model_tokenizer_class.from_pretrained("uclanlp/plbart-base", language_codes="base")
model_inputs = tokenizer([code])
print(tokenizer.decode(model_inputs['input_ids'][0], skip_special_tokens=True, clean_up_tokenization_spaces=False))
# The code output is: "public void METHOD_1 ( TYPE_1 VAR_1 ) throws .lang.Exception { super . METHOD_1 ( VAR_1 ) ; METHOD_2 ( VAR_1 ) ; }"
```

### Who can help?

@gunjan

### Information

- [ ] The official example scripts
- [ ] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

```
code = "public void METHOD_1 ( TYPE_1 VAR_1 ) throws java.lang.Exception { super . METHOD_1 ( VAR_1 ) ; METHOD_2 ( VAR_1 ) ; }"
tokenizer = model_tokenizer_class.from_pretrained("uclanlp/plbart-base", language_codes="base")
model_inputs = tokenizer([code])
print(tokenizer.decode(model_inputs['input_ids'][0], skip_special_tokens=True, clean_up_tokenization_spaces=False))
# public void METHOD_1 ( TYPE_1 VAR_1 ) throws .lang.Exception { super . METHOD_1 ( VAR_1 ) ; METHOD_2 ( VAR_1 ) ; }
```

### Expected behavior

```
code = "public void METHOD_1 ( TYPE_1 VAR_1 ) throws java.lang.Exception { super . METHOD_1 ( VAR_1 ) ; METHOD_2 ( VAR_1 ) ; }"
tokenizer = model_tokenizer_class.from_pretrained("uclanlp/plbart-base", language_codes="base")
model_inputs = tokenizer([code])
print(tokenizer.decode(model_inputs['input_ids'][0], skip_special_tokens=True, clean_up_tokenization_spaces=False))
# public void METHOD_1 ( TYPE_1 VAR_1 ) throws java.lang.Exception { super . METHOD_1 ( VAR_1 ) ; METHOD_2 ( VAR_1 ) ; }
```
cc @gchhablani, could you take a look at this?<|||||>@LysandreJik I can have a look at this if it isn't being looked at.<|||||>Please go ahead, thank you @jordiclive!<|||||>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.

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