Training of MBart
MBart is a multilingual encoder-decoder (sequence-to-sequence) model primarily intended for translation task. As the
model is multilingual it expects the sequences in a different format. A special language id token is added in both the
source and target text. The source text format is X [eos, src_lang_code] where X is the source text. The
target text format is [tgt_lang_code] X [eos]. bos is never used.
The regular [~MBartTokenizer.__call__] will encode source text format, and it should be wrapped
inside the context manager [~MBartTokenizer.as_target_tokenizer] to encode target text format.
Supervised training
```python
from transformers import MBartForConditionalGeneration, MBartTokenizer
tokenizer = MBartTokenizer.from_pretrained("facebook/mbart-large-en-ro", src_lang="en_XX", tgt_lang="ro_RO")
