Using BART for Mask Infilling makes all the first tokens missing  
I'm fine-tuning BART ```"facebook/bart-large"``` model for mask infilling. My dataset looks like below.

The original sentence BART should predict is `taste the rainbow.`, and the input data it gets is `<mask> taste <mask> rainbow <mask>`, or it should predict `global asset management`, given `<mask> global <mask> asset <mask>`. 


Generally it works well but only the first tokens are missing. BART's prediction for the first data was `aste the rainbow.` and the prediction for the second was `asset management.`. I don't know why this is happening. `taste` and `global` was given in input, why BART is missing those? Even the first token in original sentence is not given in the input BART predictions always drop the first token.  Given `<mask> happiest <mask> place <mask>`, it should predict `the happiest place on earth.`, but it gives me `happiest place on earth.`

I'm not sure this is related, but I gave `force_bos_token_to_be_generated` options to be `True`, and still not working

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

I would appreciate any help. Thanks

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.<|||||>Works fine when using the model.generate function