Problems with generating text using mbart-large-cc25
# 🐛 Bug

## Information

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

Language I am using the model on (English, Chinese ...): English, Romanian

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: (give the name)
* [x] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

I'm examining 'facebook/mbart-large-en-ro' and 'facebook/mbart-large-cc25' checkpoints of MBART.

Here is my first script translating an English sentence to Romanian:

```
from transformers import MBartTokenizer, BartForConditionalGeneration

tokenizer = MBartTokenizer.from_pretrained('facebook/mbart-large-en-ro')
model = BartForConditionalGeneration.from_pretrained('facebook/mbart-large-en-ro')

src_sent = "UN Chief Says There Is No Military Solution in Syria"

src_ids = tokenizer.prepare_translation_batch([src_sent])

output_ids = model.generate(src_ids["input_ids"], decoder_start_token_id=tokenizer.lang_code_to_id["ro_RO"])

output = tokenizer.batch_decode(output_ids, skip_special_tokens=True)[0]

print('src_sent: ', src_sent)
print('src_ids: ', src_ids)
print('output_ids: ', output_ids)
print('output: ', output)

```

stdout:

```
src_sent:  UN Chief Says There Is No Military Solution in Syria
src_ids:  {'input_ids': tensor([[  8274, 127873,  25916,      7,   8622,   2071,    438,  67485,     53,
         187895,     23,  51712,      2, 250004]]), 'attention_mask': tensor([[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]])}
output_ids:  tensor([[250020,      0,  47711,   7844, 127666,      8,  18347,  18147,   1362,
            315,  42071,     36,  31563,   8454,  33796,    451,    346, 125577]])
output:  Şeful ONU declară că nu există o soluţie militară în Siria

```

As seen in output_ids the model always generates 0 (corresponding to bos_token) at the first decoding step. However, this seems to not be a problem with this checkpoint as the output is still the correct translation.

Now I run the same script but using pretrained "facebook/mbart-large-cc25" and trying to denoise an English input. Since the input does not have mask tokens the output should be identical to the input given the pertaining objective of MBART.
However, the output always misses the first token from the input. I have observed this with different examples (even when you have masked tokens in the input).


```
from transformers import MBartTokenizer, BartForConditionalGeneration

tokenizer = MBartTokenizer.from_pretrained('facebook/mbart-large-cc25')
model = BartForConditionalGeneration.from_pretrained('facebook/mbart-large-cc25')

src_sent = "UN Chief Says There Is No Military Solution in Syria"

src_ids = tokenizer.prepare_translation_batch([src_sent])

output_ids = model.generate(src_ids["input_ids"], decoder_start_token_id=tokenizer.lang_code_to_id["en_XX"])

output = tokenizer.batch_decode(output_ids, skip_special_tokens=True)[0]

print('src_sent: ', src_sent)
print('src_ids: ', src_ids)
print('output_ids: ', output_ids)
print('output: ', output)

```

the stdout:

```
src_sent:  UN Chief Says There Is No Military Solution in Syria
src_ids:  {'input_ids': tensor([[  8274, 127873,  25916,      7,   8622,   2071,    438,  67485,     53,
         187895,     23,  51712,      2, 250004]]), 'attention_mask': tensor([[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]])}
output_ids:  tensor([[250004,      0, 127873,  25916,      7,   8622,   2071,    438,  67485,
             53, 187895,     23,  51712]])
output:  Chief Says There Is No Military Solution in Syria
```


I have tried various approaches but haven't found any clear solutions to this. Appreciate any help on this. 


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## 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: 3.0.2
- Python version: 3.7.4
- PyTorch version (GPU?): 1.4.0 (False)

Thanks for flagging. We are still trying to get to the bottom of which special tokens to use for mbart-large-cc25. See https://github.com/pytorch/fairseq/issues/2258 .<|||||>This might not be the best solution but after experimenting with the tokenizer special tokens a bit, it seems like the model is insensitive to the first input_id and lang_code used on the encoder side.
So after these modifications:

```
    def set_src_lang_special_tokens(self, src_lang) -> None:
        """Reset the special tokens to the source lang setting. No prefix and suffix=[eos, cur_lang_code]."""
        self.cur_lang_code = self.lang_code_to_id[src_lang]
        self.prefix_tokens = [self.bos_token_id]
        self.suffix_tokens = [self.eos_token_id]

    def set_tgt_lang_special_tokens(self, lang: str) -> None:
        """Reset the special tokens to the target language setting. Prefix [tgt_lang_code], suffix =[eos]."""
        self.cur_lang_code = self.lang_code_to_id[lang]
        self.prefix_tokens = [self.cur_lang_code]
        self.suffix_tokens = [self.eos_token_id]
```

in tokenization_bart.py, the model seems to be doing the right thing and generating correct English output:
```
src_sent:  UN Chief Says There Is No Military Solution in Syria
src_ids:  {'input_ids': tensor([[     0,   8274, 127873,  25916,      7,   8622,   2071,    438,  67485,
             53, 187895,     23,  51712,      2]]), 'attention_mask': tensor([[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]])}
output_ids:  tensor([[250004,      0,   8274, 127873,  25916,      7,   8622,   2071,    438,
          67485,     53, 187895,     23,  51712]])
output:  UN Chief Says There Is No Military Solution in Syria
```

Although this is not how things are described in MBART paper so the core issue remains. 

Also, this code segment in modeling_bart.py :

```
    def adjust_logits_during_generation(self, logits, cur_len, max_length, **kwargs):
        if cur_len == 1:
            self._force_token_ids_generation(logits, self.config.bos_token_id)
```
is the culprit for generating 0 (corresponding to bos_token) at every first decoding step.
It might need to be changed for MBART model.

<|||||>I find that the input does need to start with `<bos>`, and the decoder should be seeded with `<lang_code> <bos>`. With this setup, I am able to recover the input sequence during decoding. Like @Mehrad0711 I find that the input `lang_code` does not make a significant difference. <|||||>@tomhosking I replicated what you wrote. Definitely needs `<s>`(bos) at the beginning of the input string to fix the off by one error. You can see the tests in this PR #6524 .

I'm still having trouble squaring this/trying to find a unified fix to accomodate the behavior that mbart-large-en-ro seems to want, as shown in #6156 .

Maybe the simplest change is just to add `<s>` to the start of the encoder side string?<|||||>Interestingly, `prepend_bos` is set to false by default in the fairseq mbart finetuning docs.
I set a breakpoint during finetuning and there is no BOS to be found: here is [how batches look](https://gist.github.com/sshleifer/cba08bc2109361a74ac3760a7e30e4f4)<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>Hi,
Reviving this issue as it still persists in the recent version of transformers. The solution I proposed in Aug 2 comment seems to be working after a modification on prefix source token borrowed from `tokenization_mbart50.py`.
Proposed solution:
```
    def set_src_lang_special_tokens(self, src_lang) -> None:
        """Reset the special tokens to the source lang setting. Prefix=[src_lang_code] and suffix=[eos]."""
        self.cur_lang_code = self.lang_code_to_id[src_lang]
        self.prefix_tokens = [self.cur_lang_code]
        self.suffix_tokens = [self.eos_token_id]

    def set_tgt_lang_special_tokens(self, lang: str) -> None:
        """Reset the special tokens to the target language setting. Prefix=[tgt_lang_code] and suffix=[eos]."""
        self.cur_lang_code = self.lang_code_to_id[lang]
        self.prefix_tokens = [self.cur_lang_code]
        self.suffix_tokens = [self.eos_token_id]
```
This change will fix `mbart-large-cc25`'s output text while leaving `mbart-large-en-ro`'s untouched.

Code to reproduce:
```
from transformers import MBartTokenizer, MBartForConditionalGeneration

tokenizer = MBartTokenizer.from_pretrained('facebook/mbart-large-cc25')
model = MBartForConditionalGeneration.from_pretrained('facebook/mbart-large-cc25')

src_sent = "UN Chief Says There Is No Military Solution in Syria"

batch = tokenizer.prepare_seq2seq_batch(src_texts=[src_sent], src_lang="en_XX", return_tensors="pt")

output_ids = model.generate(**batch, decoder_start_token_id=tokenizer.lang_code_to_id["en_XX"])

output = tokenizer.batch_decode(output_ids, skip_special_tokens=True)[0]

print('src_sent: ', src_sent)
print('src_ids: ', batch["input_ids"])
print('output_ids: ', output_ids)
print('output: ', output)

```

stdout (before change):
```
src_sent:  UN Chief Says There Is No Military Solution in Syria
src_ids:  tensor([[  8274, 127873,  25916,      7,   8622,   2071,    438,  67485,     53,
         187895,     23,  51712,      2, 250004]])
output_ids:  tensor([[250004,      0, 127873,  25916,      7,   8622,   2071,    438,  67485,
             53, 187895,     23,  51712,      2]])
output:  Chief Says There Is No Military Solution in Syria
```

stdout (after change):
```
src_sent:  UN Chief Says There Is No Military Solution in Syria
src_ids:  tensor([[250004,   8274, 127873,  25916,      7,   8622,   2071,    438,  67485,
             53, 187895,     23,  51712,      2]])
output_ids:  tensor([[250004,      0,   8274, 127873,  25916,      7,   8622,   2071,    438,
          67485,     53, 187895,     23,  51712,      2]])
output:  UN Chief Says There Is No Military Solution in Syria

```

Potential reviewers: @patrickvonplaten, @patil-suraj, @sgugger 
<|||||>Hi @sgugger, @patrickvonplaten, @patil-suraj,
it would be great if you could provide your feedback on this. I would be happy to provide more context if needed.<|||||>Hi @Mehrad0711 

Thank you for reporting this.  I'll go through the original model code in fairseq to see how they are handling the prefix tokens and get back here.<|||||>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,
Was wondering if you had the chance to take a look at this. Thanks.<|||||>Hi @Mehrad0711 

You are right, all mBART models actually use the language code as the prefix token and `<eos>` as the suffix token.
But unfortunately, we can't really change it now, because this will be backward incompatible with the other models trained using the existing format.

Also, this doesn't really make that much difference if you want to fine-tune the model. As long as a consistent format is used for fine-tuning and then for inference then it should work. However, it would change the output for the pre-trained models (as you reported). But as `mbart-large-cc25`  is just a pre-trained model and should be fine-tuned to use it for the downstream tasks, this doesn't seem like a big issue.
<|||||>Hi @patil-suraj!
Thanks for your reply. I understand the concern regarding backward incompatibility of this change.
I was using `mbart-large-cc25` without fine-tuning for text denoising; that's how the problem popped up. Given newer mbart models are now available on huggingface, I'll switch to them.