BART infilling example? 
Hi, 

I'm trying this official example from the documentation (v4.7.0) for BART mask infilling :

 "https://huggingface.co/transformers/model_doc/bart.html#bartforconditionalgeneration"

```
from transformers import BartForConditionalGeneration, BartTokenizer
model = BartForConditionalGeneration.from_pretrained("facebook/bart-large", force_bos_token_to_be_generated=True)
tok = BartTokenizer.from_pretrained("facebook/bart-large")
example_english_phrase = "UN Chief Says There Is No <mask> in Syria"
batch = tok(example_english_phrase, return_tensors='pt')
generated_ids = model.generate(batch['input_ids'])
assert tok.batch_decode(generated_ids, skip_special_tokens=True) == ['UN Chief Says There Is No Plan to Stop Chemical Weapons in Syria']
```

First, I get this `TypeError: __init__() got an unexpected keyword argument 'force_bos_token_to_be_generated`
After removing that argument, the output is:` ['UNALSO SEE']`, which is unexpected. 

I also tried several other examples but get behavior which does not seem like infilling:

```
example_english_phrase = "They are having a <mask> in a park."
batch = tok(example_english_phrase, return_tensors='pt')
generated_ids = model.generate(batch['input_ids'])
tok.batch_decode(generated_ids, skip_special_tokens=True)  

```
Output: `"'They are in a park.They are having a party.'"`



**Do I have the wrong model or documentation?** Grateful for any pointers to help resolve this. 

@patrickvonplaten, @patil-suraj
- `transformers` version: 4.7.0
- Python version: 3.7
- PyTorch version (GPU?): 1.7.1+cu110


(Resolved,  but documentation needs to be updated)

```
from transformers import BartConfig
config = BartConfig(force_bos_token_to_be_generated=True)
model = BartForConditionalGeneration.from_pretrained("facebook/bart-large", config=config)
...
```

will let the model do infilling. <|||||>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.