Add config option to skip 1-D position embeddings in LayoutLM
# 🚀 Feature request
Add an option in LayoutLM config to not use 1-D position embeddings. The config currently allows us to choose between "absolute", "relative_key", and "relative_key_query". Can we add another option like "none" to not use 1-D positional embeddings?

## Motivation
The input to LayoutLM consists of tokens of text and their corresponding bounding boxes from document images. This is typically obtained by passing the document image through an OCR. 

LayoutLM uses 1-D as well as 2-D position embeddings. While OCRs provide reliable 2-D positions for each word in the document image, the order of words (1-D positions) are not always correct. For example, if we OCR a two column document, or a document containing a table, or any other visually rich document, the order of words in the OCR output is very unreliable. This unreliable position information harms accuracy in several downstream tasks. I have personally seen improvements in some tasks when I manually disable 1-D position embeddings in LayoutLM, and force the model to only look at the 2-D positions. Can we provide an easy way to do this by adding an option in the LayoutLM config to make 1-D position embeddings optional?

## Your contribution
I am willing to work on this and submit a PR, but this is the first time I am contributing to the library and might require some help.

I'm not sure whether we should add such an option, because models like BERT, RoBERTa, ... basically all Transformer models within this repository don't have this option. Either we add that option to all, either we don't in my opinion. 

Absolute position embeddings are almost always beneficial, so not sure if adding this will have value, perhaps we could strive for simplicity. cc @sgugger @LysandreJik <|||||>Agreed. Unless there are pretrained checkpoints available that require this kind of change, you should just tweak the code of `modeling_layoutlm` to your needs for this.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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