Pegasus tokenizer does not have bos token, cannot pretrain
## 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:
- Platform: Ubuntu 18.04
- Python version: 3.8
- PyTorch version (GPU?): 1.7.1 with GPU (CUDA 10.1)
- Tensorflow version (GPU?): N/A
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No

### Who can help

@patrickvonplaten @LysandreJik


## Information

Model I am using (Bert, XLNet ...): Pegasus

The problem arises when using:
* [ ] the official example scripts: (give details below)
* [x] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: SQUaD
* [x] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

I am trying to re-create the basic objective of pre-training with Pegasus. 

I believe the issue is with the bos token: it does not exist, as per this PR: https://github.com/huggingface/transformers/pull/8731/files. However, it does exist in the [original paper](https://arxiv.org/pdf/1912.08777.pdf) (it's `<s>`)

Steps to reproduce:

```
model_name = 'google/pegasus-cnn_dailymail'
from transformers import PegasusForConditionalGeneration, PegasusTokenizer
model = PegasusForConditionalGeneration.from_pretrained(model_name)
## Taken from paper:
input_string = ["Pegasus is <mask_2> . <mask_1> it <mask_2> the model ."]
input_ids = tokenizer(input_string, add_special_tokens=False, return_tensors="pt").input_ids
print(input_ids) ## tensor([[51881,   117,     3,   110,   107,     2,   126,     3,   109,   861, 110,   107]])

decoder_input_string = ["<s> It is pure white . "]
decoder_input_ids = tokenizer(decoder_input_string, add_special_tokens=False, return_tensors="pt", bos_token='<s>').input_ids
print(decoder_input_ids) ## tensor([[ 110,  105,  116, 2314,  168,  117, 3763,  695,  110,  107]])

labels_string = ["It is pure white . </s>"]
labels = tokenizer(labels_string, add_special_tokens=False, return_tensors="pt").input_ids
print(labels) ## tensor([[ 168,  117, 3763,  695,  110,  107,    1]])

loss = model(input_ids=input_ids, decoder_input_ids=decoder_input_ids, labels=labels)[0]
```

In the final line, I get the following error:

```
ValueError                                Traceback (most recent call last)
<ipython-input-32-13dda8f18c44> in <module>
----> 1 loss = model(input_ids=input_ids, decoder_input_ids=decoder_input_ids, labels=labels)[0]

/home/ubuntu/anaconda3/envs/pytorch_new/lib/python3.8/site-packages/torch/nn/modules/module.py in _call_impl(self, *input, **kwargs)
    725             result = self._slow_forward(*input, **kwargs)
    726         else:
--> 727             result = self.forward(*input, **kwargs)
    728         for hook in itertools.chain(
    729                 _global_forward_hooks.values(),

/home/ubuntu/anaconda3/envs/pytorch_new/lib/python3.8/site-packages/transformers/models/pegasus/modeling_pegasus.py in forward(self, input_ids, attention_mask, decoder_input_ids, decoder_attention_mask, head_mask, decoder_head_mask, encoder_outputs, past_key_values, inputs_embeds, decoder_inputs_embeds, labels, use_cache, output_attentions, output_hidden_states, return_dict)
   1285         if labels is not None:
   1286             loss_fct = CrossEntropyLoss()
-> 1287             masked_lm_loss = loss_fct(lm_logits.view(-1, self.config.vocab_size), labels.view(-1))
   1288 
   1289         if not return_dict:

/home/ubuntu/anaconda3/envs/pytorch_new/lib/python3.8/site-packages/torch/nn/modules/module.py in _call_impl(self, *input, **kwargs)
    725             result = self._slow_forward(*input, **kwargs)
    726         else:
--> 727             result = self.forward(*input, **kwargs)
    728         for hook in itertools.chain(
    729                 _global_forward_hooks.values(),

/home/ubuntu/anaconda3/envs/pytorch_new/lib/python3.8/site-packages/torch/nn/modules/loss.py in forward(self, input, target)
    959 
    960     def forward(self, input: Tensor, target: Tensor) -> Tensor:
--> 961         return F.cross_entropy(input, target, weight=self.weight,
    962                                ignore_index=self.ignore_index, reduction=self.reduction)
    963 

/home/ubuntu/anaconda3/envs/pytorch_new/lib/python3.8/site-packages/torch/nn/functional.py in cross_entropy(input, target, weight, size_average, ignore_index, reduce, reduction)
   2466     if size_average is not None or reduce is not None:
   2467         reduction = _Reduction.legacy_get_string(size_average, reduce)
-> 2468     return nll_loss(log_softmax(input, 1), target, weight, None, ignore_index, None, reduction)
   2469 
   2470 

/home/ubuntu/anaconda3/envs/pytorch_new/lib/python3.8/site-packages/torch/nn/functional.py in nll_loss(input, target, weight, size_average, ignore_index, reduce, reduction)
   2259 
   2260     if input.size(0) != target.size(0):
-> 2261         raise ValueError('Expected input batch_size ({}) to match target batch_size ({}).'
   2262                          .format(input.size(0), target.size(0)))
   2263     if dim == 2:

ValueError: Expected input batch_size (10) to match target batch_size (7).
```

it seems to me like the issue is while tokenizing `decoder_input_ids`: the `<s>` gets tokenized as 4 different indexes `110,  105,  116, 2314` instead of just one. This is because there is no bos_token in the tokenizer. 

## Expected behavior

ValueError should not be thrown and the `decoder_input_ids` should have same length as `labels`, allowing `model(...)` call to work correctly.


Hi there,

yes, PEGASUS has no BOS token, instead it uses `pad_token_id` as the `decoder_start_token_id`.  Also, you won't need to pass `deocder_input_ids`, if you pass the `labels` the model prepares the `decoder_input_ids` by shifting the `labels` and setting the first token to pad token.<|||||>Hi @patil-suraj, thanks for answering! Do you mind sharing a small working snippet? I'm new to this library and this model in particular is a bit difficult to understand. <|||||>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.<|||||>Hi @patil-suraj . Could you please provide a small working snippet of the code you mentioned? <|||||>Hi @adivekar-utexas  sorry to only answer now.

```python
from transformers import PegasusTokenizer

tokenizer = PegasusTokenizer.from_pretrained("google/pegasus-large")

src_text = "This is source text"
tgt_text = "This is target text"

inputs = tokenizer(src_text, return_tensors="pt")
inputs["labels"] = tokenizer(tgt_text, return_tensors="pt").input_ids

outputs = model(**inputs)
```<|||||>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.<|||||>Hi I am trying to use GSG under Bart environment. If I understand right about the GSG, there must be multiple target sentences not one unlike above example. 

So, is it right that the shape of decoder input and label shape as below(It there are 2 masked sentences)?

- decoder input  string : \<pad> It is the first masked sentence.\<pad>It is the second masked sentence.
- decoder label string : It is the first masked sentence.\<pad>It is the second masked sentence.\</s>