src_lang/tgt_lang missing in mbart example
## 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?
Tagging @patrickvonplaten @patil-suraj @LysandreJik again in case you know what was going on here. <|||||>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.<|||||>Hi @zijwang , thanks a lot for spotting this.

The tokenizer API is changed a bit,  instead of passing `src_lang` and `tgt_lang` to tokenizes `__call__` method, we can now pass these when initializing the tokenizer, or we could set those properties as well. Here's a minimal example 

```python
from transformers import MBartForConditionalGeneration, MBartTokenizer

tokenizer = MBartTokenizer.from_pretrained("facebook/mbart-large-en-ro", src_lang="en_XX", tgt_lang="ro_RO")

# to change the src_lang
tokenizer.src_lang = "fr_XX"
```