[Pegasus][tokenizer] pegasus tokenizer doesn't have any BOS token?
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: 4.9.0.dev0
- Platform: Ubuntu
- Python version: 3.8
- PyTorch version (GPU?): 1.7 (yes)
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: Yes

@patrickvonplaten, @patil-suraj, @LysandreJik


## Information

Model I am using (PEGASUS ...):

This is not specifically a bug in transformers, but I'm wondering if I can comment and ask my question in this section as there was no other section I could relate this to.

I'm trying to use pegasus's encoder for extractive summarization proposes so that system can identify top sentences. To do so, I want to implement an approach like BERTSUM where [CLS] tokens are being added to the beginning of each sentence. 

Talking of pegasus, I noticed there's no so-called BOS (similar to CLS head in BERT) token which denotes the start of each sentence (below) –although it's available in BART's tokenizer.

https://github.com/huggingface/transformers/blob/master/src/transformers/models/pegasus/tokenization_pegasus.py#L102

```
...
def __init__(
        self,
        vocab_file,
        pad_token="<pad>",
        eos_token="</s>",
        unk_token="<unk>",
        mask_token="<mask_2>",
        mask_token_sent="<mask_1>",
        additional_special_tokens=None,
        offset=103,  # entries 2 - 104 are only used for pretraining
        sp_model_kwargs: Optional[Dict[str, Any]] = None,
        **kwargs
    ) -> None:
...
```

However, there's `additional_special_tokens` by which I can define new special tokens (like the one I intend to use for sentences). Is that possible? I mean, is the new token expected to give representations of what tokens (i.e.,. sentences) following it? Thanks!
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