Add VisualBERT demo notebook
In continuation with #10534, this PR adds demo for VisualBERT model.
I am planning to base it on the `LXMERT` examples, hence the copy-paste of files for now.
I have updated the demo. Turns out I didn't need to change a lot from the LXMERT demo. I have used the same files, just replaced the tokenizer, the model and the labels that are being used. Only `demo.ipynb` is different.

Requesting @LysandreJik @patil-suraj to review.<|||||>Thanks for approving and merging @patil-suraj @LysandreJik ^_^ 