Support for new LayoutLMv2 in Auto Classes (specifically AutoModelForMaskedLM)
# 🚀 Feature request

I am trying to finetune a LayoutLMv2 language model on my dataset using the Auto Class functionality, namely AutoModelForMaskedLM. However, it does not support this model. (https://huggingface.co/transformers/model_doc/auto.html#automodelformaskedlm) 

## Motivation

It is hard to develop a streamlined and consistent pipeline if I have to jump back and forth between ways to finetune depending on the model I use. 
As the authors did not release any pretraining code, we did not implement a `LayoutLMv2ForMaskedLM`, as LayoutLMv2 was actually pre-trained on 3 tasks:

* Masked Visual-Language Modeling
* Text-Image Alignment
* Text-Image Matching

Of course, we could add a `LayoutLMv2ForMaskedLM`, which adds a language modeling head on top of the base model. This could be used to replicate the first pre-training objective. Feel free to open a PR if you would like this to be added.
<|||||>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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