MBART pretrained model is unable to produce output in the target language
Hi,

I am using mbart-large-50 for generation task. Source language is Hindi and target language is Gujarati. However, I am always getting the output in Hindi. It is expected to get few tokens in the target language even though its a pretrained model since i am forcing the BOS token to the target language. 

Sharing the code that i am using for this task.

`
# translate Hindi to Gujarati
from transformers import MBartForConditionalGeneration, MBart50TokenizerFast
model = MBartForConditionalGeneration.from_pretrained("facebook/mbart-large-50")
tokenizer = MBart50TokenizerFast.from_pretrained("facebook/mbart-large-50")

tokenizer.src_lang = "hi_IN"
article_hi = "संयुक्त राष्ट्र के प्रमुख का कहना है कि सीरिया में कोई सैन्य समाधान नहीं है"
encoded_hi = tokenizer(article_hi, return_tensors="pt")
generated_tokens = model.generate(**encoded_hi, forced_bos_token_id=tokenizer.lang_code_to_id["gu_IN"])
tokenizer.batch_decode(generated_tokens, skip_special_tokens=True)`


@patrickvonplaten
You also need to set the `tokenizer.tgt_lang` I believe.
Also cc @ArthurZucker <|||||>I think you are just using the wrong checkpoint. 
Using the `"facebook/mbart-large-50-many-to-many-mmt"` I obtain the following : 
```યુનાઇટેડ સ્ટેટ્સ ઓફ અમેરિકાના પ્રાંતિકારી کہتے हैं कि सीरिया में कोई सैन्य समाधान नहीं है```
which, according to Google is Gujarati!. <|||||>@ArthurZucker "facebook/mbart-large-50-many-to-many-mmt" is fine tuned checkpoint. I am trying with a pretrained checkpoint which is "facebook/mbart-large-50". 

The pretrained checkpoint should also be able to give output in the target language if we force the BOS token to the target language. The output may be little bit distorted but that's fine.  Here, its giving the output same as the source language. <|||||>> The pretrained checkpoint should also be able to give output in the target language if we force the BOS token to the target language 

I think this depends on the language since it is a `pretrained checkpoint` as mentioned on the model card : 
> `mbart-large-50` is pre-trained model and primarily aimed at being fine-tuned on translation tasks. It can also be fine-tuned on other multilingual sequence-to-sequence tasks. See the [model hub](https://huggingface.co/models?filter=mbart-50) to look for fine-tuned versions.

Since it works totally fine with the fine-tuned checkpoint, this is not a bug.<|||||>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/main/CONTRIBUTING.md) are likely to be ignored.<|||||>Hi, is there some mismatch between the tokenizer of `facebook/mbart-large-50` and `shift_tokens_right` of `MBartForConditionalGeneration`? Since the tokenizer of `facebook/mbart-large-en-ro` would give **X [eos, src_lang_code]** while `facebook/mbart-large-50`'s tokenizer would give **[src_lang_code] X [eos]**, but they both use the same `shift_tokens_right` method which I believe is only suitable for input like this **X [eos, src_lang_code]** :
```python

def shift_tokens_right(input_ids: torch.Tensor, pad_token_id: int):
    """
    Shift input ids one token to the right, and wrap the last non pad token (the <LID> token) Note that MBart does not
    have a single `decoder_start_token_id` in contrast to other Bart-like models.
    """
    prev_output_tokens = input_ids.clone()

    if pad_token_id is None:
        raise ValueError("self.model.config.pad_token_id has to be defined.")
    # replace possible -100 values in labels by `pad_token_id`
    prev_output_tokens.masked_fill_(prev_output_tokens == -100, pad_token_id)

    index_of_eos = (prev_output_tokens.ne(pad_token_id).sum(dim=1) - 1).unsqueeze(-1)
    decoder_start_tokens = prev_output_tokens.gather(1, index_of_eos).squeeze()
    prev_output_tokens[:, 1:] = prev_output_tokens[:, :-1].clone()
    prev_output_tokens[:, 0] = decoder_start_tokens

    return prev_output_tokens
```<|||||>Indeed. But as mentioned in the documentation : 
> The text format for MBart-50 is slightly different from mBART. For MBart-50 the language id token is used as a prefix for both source and target text i.e the text format is [lang_code] X [eos], where lang_code is source language id for source text and target language id for target text, with X being the source or target text respectively.
While 
> For MBart [...] the source text format is X [eos, src_lang_code] where X is the source text. The target text format is [tgt_lang_code] X [eos]. bos is never used.

Which is why they don't have the same tokenization scheme.
I checked that when generating, the `forced_decoder_id` properly works, and I think this issue can be closed as there are no guarantee that a certain pair of language will produce intelligible result as the checkpoints are pretrained. 


<|||||>Hi, thanks for the comments!
It is true that using MBart-50 to do generation with proper `forced_decoder_id` works. But it doesn't work on supervised learning scenarios. When there is no `decoder_input_ids` for training, Mbart-50 would automatically create`decoder_input_ids` from `labels` which follows the tokenization scheme of Mbart rather than Mbart-50.  And I think this should be fixed.
<img width="770" alt="MBart and MBart-50 2023-01-30 17-49-21" src="https://user-images.githubusercontent.com/38466901/215444119-90199c9d-baa2-421d-86be-0d0e4e585e2c.png">

<|||||>I am not sure I understand. When the `decoder_input_ids` are created from the `labels`, they are a shifted version. 
Let's use the example: 
- src_text : `'en_XX UN Chief Says There Is No Military Solution in Syria</s>'`
- labels : `'ro_RO Şeful ONU declară că nu există o soluţie militară în Siria</s>'`
- [shifted labels](https://github.com/huggingface/transformers/blob/main/src/transformers/models/mbart/modeling_mbart.py#L1348-L1349) : `'</s>ro_RO Şeful ONU declară că nu există o soluţie militară în Siria'` (= decoder_inputs_ids)
This means that the `shifted_labels` will follow the correct pattern (which you enforce when generating). 


<|||||>Sorry, my bad. You are right. I mistakenly thought the generation schema of MBart-50 is the same as MBart, whose `decoder_start_token_id` is the `lang_id`.