Fix doc examples: KeyError
# What does this PR do?

The doc examples in `BlenderbotSmall` have

```
        >>> inputs = tokenizer([UTTERANCE], return_tensors='pt')
        >>> inputs.pop("token_type_ids")
```
However, `BlenderbotSmallTokenizer` doesn't give `token_type_ids`, and cause `KeyError` here.

This PR fix this.

## Who can review?


@patrickvonplaten @patil-suraj