Issue with mBART50 es-en translation 
## Environment info
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- `transformers` version: 4.6.1
- Platform: colab
- Python version: 3.7
- PyTorch version (GPU?): 
- Tensorflow version (GPU?):
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: No

### Who can help
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @Rocketknight1

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

Model hub:

- for issues with a model report at https://discuss.huggingface.co/ and tag the model's creator.

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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## Information

Model I am using (Bert, XLNet ...): mBART-large-50-many-to-one-nmt

The problem arises when using:
* [x] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is: Translation
* [x] an official GLUE/SQUaD task: (give the name)
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behaviour:

The below notebook can be used to reproduce the results
1. https://colab.research.google.com/drive/1LEY3bI9mS7D-n6rJ70iKq3lN9_DQCQh7?usp=sharing

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

I've used this model to translate a lot of Spanish text. But I observed that for some examples it's printing completely random things. 

The above example should return something like this `1980 Mount St. Helens eruption`

The current output is `The Committee recommends that the State party take all necessary measures to ensure the full implementation of the present recommendations, inter alia, by transmitting them to the members of the Council of Ministers, the Parliament, the Parliamentary Assembly and the Senate, the Parliamentary Assembly and the National Assembly, for appropriate consideration and further action.`

Tagging @patrickvonplaten, @patil-suraj here. I believe this is not really a code issue, but something intrinsic to the model. Any ideas why this is happening?

Ping <|||||>Hi @patrickvonplaten any thing you wanted to check. Sorry for the late response, was a bit tied up<|||||>@patil-suraj - seems like multiple people have problems with mBART50...should we maybe leave a note in the official docs about it? 