Remove version warning in pretrained BART models
# What does this PR do?

This PR fixes the warnings when loading any pretrained BART model:
```
Some weights of the model checkpoint at facebook/bart-large-mnli were not used when initializing BartModelForSequenceClassification: ['model.encoder.version', 'model.decoder.version']
- This IS expected if you are initializing BartModelForSequenceClassification from the checkpoint of a model trained on another task or with another architecture (e.g. initializing a BertForSequenceClassification model from a BertForPreTraining model).
- This IS NOT expected if you are initializing BartModelForSequenceClassification from the checkpoint of a model that you expect to be exactly identical (initializing a BertForSequenceClassification model from a BertForSequenceClassification model).
```