[Enhancement] Joint entity recognition and page/document classification

[athewsey]

Today we demonstrate annotation and training for entity extraction only. For many use cases document classification is also important, and it should be pretty straightforward to support this too.

A sequence classification task is already supported in the open source (e.g. LayoutLMv2ForSequenceClassification), but joint cls+ner with a single model might be performant and more economical for users - and not require too much extra effort.