Problem about using mBART50 for Russian to Chinese translation
## 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: 4.9.1
- Platform: ubuntu 18.04 
- Python version: 3.6.9. 
- PyTorch version (GPU?): 1.8.0 
- Tensorflow version (GPU?): 2.3.0 
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no 

### Who can help

mbart-large-50-many-to-many-mmt：@LysandreJik @patrickvonplaten  

## Information

Model I am using: mbart-large-50-many-to-many-mmt

The problem arises when using:  
  
* my own modified scripts: (give details below)  

We originally wanted to do a Russian-Chinese translation task, but our translation results showed a lot of English. We used a script to test.

## To reproduce

Steps to reproduce the behavior:

1.The code is as follow:  
```python
from transformers import MBartForConditionalGeneration, MBart50TokenizerFast

text_list = ['Это позволит облегчить транспортировку грузов для Китая и Германии.',
             'Россия останется одним из лидеров, возможности для наращивания экспорта есть.',
             'Это позволит оптимизировать торговые отношения.']

src_lang = 'ru_RU'
tgt_lang = 'zh_CN'
model = MBartForConditionalGeneration.from_pretrained("facebook/mbart-large-50-many-to-many-mmt")
tokenizer = MBart50TokenizerFast.from_pretrained("facebook/mbart-large-50-many-to-many-mmt")


for text in text_list:
    tokenizer.src_lang = src_lang
    encoded_hi = tokenizer(text, return_tensors="pt")
    generated_tokens = model.generate(**encoded_hi, forced_bos_token_id=tokenizer.lang_code_to_id[tgt_lang])
    translated = tokenizer.batch_decode(generated_tokens, skip_special_tokens=True)
    print(translated)
```  
when the src_lang was ‘ru_RU’ and the tgt_lang was ‘zh_CN’, the results were: 
```
['This will facilitate the transport of goods for China and Germany.']
['Russia will remain one of the leaders, there are opportunities to increase export.']
['This will allow to optimize trade relations.']
```


<!-- If you have code snippets, error messages, stack traces please provide them here as well.
     Important! Use code tags to correctly format your code. See https://help.github.com/en/github/writing-on-github/creating-and-highlighting-code-blocks#syntax-highlighting
     Do not use screenshots, as they are hard to read and (more importantly) don't allow others to copy-and-paste your code.-->

## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
We wanted to obtain a set of Chinese translations. Here are the Chinese translations for reference.
```
['这将使中国和德国更容易运输货物。']
['俄罗斯仍将是一个领导者，有机会增加出口。']
['这将有助于改善贸易关系。']
```


Same problem, please have a look~ @patil-suraj<|||||>Yes I've seen similar issues with mBART50 returning random sentences as output. Related issues are #12104 and #12958 <|||||>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.<|||||>I don't think this was fixed<|||||>@patil-suraj - pinging again here. Would be great to take some action here soon<|||||>Sorry about being super slow here.

Going to take look at it this week. First step is to do the same generation with the original model [here](https://github.com/pytorch/fairseq/tree/main/examples/multilingual), the setup is very complicated. Will do it and post the instructions here as well. If the generations match then the issue is with the model itself.
<|||||>is there a translation from English to Chinese ?
Or From Chinese to English ? 