

TITLE
Does MBartTokenizer remove the parameter decoder_input_ids?

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## 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:3.4.0
- Platform:Google Colab
- Python version:3.7
- PyTorch version (GPU?):1.7.0+cu101
- Tensorflow version (GPU?):2.x
- Using GPU in script?: no
- Using distributed or parallel set-up in script?:no

### Who can help
<!-- Your issue will be replied to more quickly if you can figure out the right person to tag with @
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 albert, bert, GPT2, XLM: @LysandreJik 
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Speed and Memory Benchmarks: @patrickvonplaten
 Model Cards: @julien-c
 Translation: @sshleifer
 Summarization: @sshleifer
 TextGeneration: @TevenLeScao 
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
 rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)
 Text Generation: @patrickvonplaten @TevenLeScao
 blenderbot: @mariamabarham
 Bart: @sshleifer
 Marian: @sshleifer
 T5: @patrickvonplaten
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 TransfoXL/XLNet: @TevenLeScao
 RAG: @patrickvonplaten, @lhoestq
 FSTM: @stas00
 examples/seq2seq: @sshleifer
 examples/bert-loses-patience: @JetRunner
 tensorflow: @jplu
 examples/token-classification: @stefan-it
 documentation: @sgugger
 -->

## 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)

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
```python
example_english_phrase = "UN Chief Says There Is No Military Solution in Syria"
expected_translation_romanian = "Şeful ONU declară că nu există o soluţie militară în Siria"
batch = MBartTokenizer.from_pretrained('facebook/mbart-large-cc25').prepare_seq2seq_batch(example_english_phrase, src_lang="en_XX", tgt_lang="ro_RO", tgt_texts=expected_translation_romanian)
input_ids = batch["input_ids"]
target_ids = batch["decoder_input_ids"]
```
Steps to reproduce the behavior:
```python
KeyError                                  Traceback (most recent call last)
<ipython-input-11-b3eedaf10c3e> in <module>()
      3 batch = MBartTokenizer.from_pretrained('facebook/mbart-large-en-ro').prepare_seq2seq_batch(example_english_phrase, src_lang="en_XX", tgt_lang="ro_RO", tgt_texts=expected_translation_romanian)
      4 input_ids = batch["input_ids"]
----> 5 target_ids = batch["decoder_input_ids"]

/usr/local/lib/python3.6/dist-packages/transformers/tokenization_utils_base.py in __getitem__(self, item)
    232         """
    233         if isinstance(item, str):
--> 234             return self.data[item]
    235         elif self._encodings is not None:
    236             return self._encodings[item]

KeyError: 'decoder_input_ids'
```

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