mBART and DataCollatorForLanguageModeling: index -1 is out of bounds for dimension 1 with size N
## Environment info
- `transformers` version: 4.5.1
- Platform: Linux-3.10.0-1062.9.1.el7.x86_64-x86_64-with-centos-7.7.1908-Core
- Python version: 3.6.8
- PyTorch version (GPU?): 1.7.1+cu101 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
@patrickvonplaten

## Information

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

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

I would like to finetune mBART with a parallel corpus (custom dataset). 

Steps to reproduce the behavior:

1. Load an MBartForConditionalGeneration and an MBartTokenizer with pre-trained weights.
2. DataCollatorForLanguageModeling as the data collator.
3. I use a Trainer and a custom torch Dataset class.

I noticed the problem arises when combining mBART and DataCollatorForLanguageModeling. I found a similar issue without no specific solution: https://github.com/huggingface/transformers/issues/9417. Here is my error stack:

```
 File "main.py", line 99, in <module>
    main()
  File "main.py", line 85, in main
    resume_from_checkpoint=LANG_MODEL_PATH + 'last_model' if os.path.exists(LANG_MODEL_PATH + 'last_model') else None
  File "/var/python3envs/transformers-4.5.1/lib/python3.6/site-packages/transformers/trainer.py", line 1120, in train
    tr_loss += self.training_step(model, inputs)
  File "/var/python3envs/transformers-4.5.1/lib/python3.6/site-packages/transformers/trainer.py", line 1524, in training_step
    loss = self.compute_loss(model, inputs)
  File "/var/python3envs/transformers-4.5.1/lib/python3.6/site-packages/transformers/trainer.py", line 1556, in compute_loss
    outputs = model(**inputs)
  File "/var/python3envs/transformers-4.5.1/lib/python3.6/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/var/python3envs/transformers-4.5.1/lib/python3.6/site-packages/transformers/models/mbart/modeling_mbart.py", line 1287, in forward
    decoder_input_ids = shift_tokens_right(labels, self.config.pad_token_id)
  File "/var/python3envs/transformers-4.5.1/lib/python3.6/site-packages/transformers/models/mbart/modeling_mbart.py", line 74, in shift_tokens_right
    decoder_start_tokens = prev_output_tokens.gather(1, index_of_eos).squeeze()
RuntimeError: index -1 is out of bounds for dimension 1 with size 309
 33%|███████████████                              | 1/3 [00:29<00:58, 29.07s/it]
```
Hi @AdrianNunez 

Could you post the for of the `input_ids` and `labels`. As explained in the [docs](https://huggingface.co/transformers/model_doc/mbart.html#training-of-mbart), mBART expects `input_ids` and `labels` in a certain format. 

`labels` are prepared with the format `ids [eos, tgt_lang_code]` and then the `shift_tokens_right` function prepares `decoder_input_ids` by shifting the `labels` to right so `decoder_input_ids` become `[tgt_lang_code] ids [eos]`

so from the error, it seem that there is either `eos` or `tgt_lang_code` code missing in the labels. But if this is how you want to use it then you should provide the `deocder_input_ids` manually.

Also `DataCollatorForLanguageModeling` is not really meant to be used with mBART, it's intended MLM and auto-regressive models like BERT and GPT, so it might not work with mBART which is expected. <|||||>Hi @patil-suraj, thank you for your answer. This is an input example:

```
{'input_ids': tensor([ 33424,      6,  95866, 216479,    104,   3934,     10,   5744,     41,
            22,      6,      4,     10,  23182,      6,      5,      2, 250004,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1]), 
'special_tokens_mask': tensor([0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
        1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]), 
'attention_mask': tensor([1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]), 
'labels': tensor([  3786,  11281,    293,  13173,  90929,     23,      6,      4,    293,
         19190,  59486,      6,      5,      2, 250019,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1,      1,      1,      1,      1,      1,      1,      1,
             1,      1])}
```

And decoding the input and labels:

```
['Beat', '', 'rix', 'evolve', 'd', 'into', 'a', 'modern', 'que', 'en', '', ',', 'a', 'professional', '', '.', '</s>', 'en_XX', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>']
['Ze', 'werd', 'een', 'moderne', 'koning', 'in', '', ',', 'een', 'vak', 'vrouw', '', '.', '</s>', 'nl_XX', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>', '<pad>']
```

> Also DataCollatorForLanguageModeling is not really meant to be used with mBART, it's intended MLM and auto-regressive models like BERT and GPT, so it might not work with mBART which is expected.

Thank you for the advice. Is there an specific data collator for the BART model family?

Thank you in advance.<|||||>Thanks, I will take a look.

> Thank you for the advice. Is there an specific data collator for the BART model family?

If you want to train mBART for [translation](https://github.com/huggingface/transformers/tree/master/examples/pytorch/translation) or [summrization](https://github.com/huggingface/transformers/tree/master/examples/pytorch/summarization) you could take a look at these examples <|||||>> If you want to train mBART for translation or summrization you could take a look at these examples

Thank you for the links and the help. <|||||>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.