

TITLE
src_lang/tgt_lang missing in mbart example

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## Environment info

- `transformers` version: 4.6
- Platform: linux
- Python version: 3.8
- PyTorch version (GPU?): 1.8

### Who can help

Models:

- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj

Library:
- tokenizers: @LysandreJik

## Information

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

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


## To reproduce

I am running the official example in the doc [here](https://huggingface.co/transformers/model_doc/mbart.html) under `Supervised training`. However, there is a warning of 

```
Keyword arguments {'src_lang': 'en_XX', 'tgt_lang': 'ro_RO'} not recognized.
```

when running

```
inputs = tokenizer(example_english_phrase, return_tensors="pt", src_lang="en_XX", tgt_lang="ro_RO")
```

Is this normal?