Fix LayoutLM ONNX test error
# What does this PR do?

Fixes the batch_size and seq_len computation during ONNX export in configuration_layoutlm.py.

PRs: #13702, #13562
Issue: #13300

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- [ ] Did you write any new necessary tests?


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@michaelbenayoun @LysandreJik @NielsRogge @mfuntowicz 
Hi,

Can you also add LayoutLM to the list of supported models? https://huggingface.co/transformers/serialization.html

Thanks!<|||||>Seems good to me!

@NielsRogge I think [he has already done it](https://github.com/huggingface/transformers/pull/13562/files#diff-f4bad33d844c5d91b09dd0af27395ade640e907920b357daf125dd19458bdd60R82), or you're talking about something different?<|||||>Yeah I'm talking about the documentation (.rst file): https://github.com/huggingface/transformers/blob/master/docs/source/serialization.rst

It also seems like other models (like mBART) are supported to be exported to ONNX, but they are not mentioned in the docs. I asked @sgugger if we can create an automagically updated table for this.<|||||>@NielsRogge I have added LayoutLM to the list of supported models. :)