Add visualbert in visual question answering model mapping
Following the doc https://huggingface.co/docs/transformers/v4.25.1/en/model_doc/visual_bert#transformers.VisualBertForQuestionAnswering I reckon this one was missing.

## Who can review?
@ydshieh do I need to write any additional test for this?

No, but we should check if the pipeline testing (in the PR CI job page) runs against this newly added `visual_bert`.
<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>VisualBERT isn't supported by the VQA pipeline, the pipeline is currently very specifically implemented to work with ViLT.