Add LayoutLMForQuestionAnswering model (#18407)
* Add LayoutLMForQuestionAnswering model
* Fix output
* Remove TF TODOs
* Add test cases
* Add docs
* TF implementation
* Fix PT/TF equivalence
* Fix loss
* make fixup
* Fix up documentation code examples
* Fix up documentation examples + test them
* Remove LayoutLMForQuestionAnswering from the auto mapping
* Docstrings
* Add better docstrings
* Undo whitespace changes
* Update tokenizers in comments
* Fixup code and remove `from_pt=True`
* Fix tests
* Revert some unexpected docstring changes
* Fix tests by overriding _prepare_for_class
Co-authored-by: Ankur Goyal <ankur@impira.com>
