Removing BertConfig inheritance from LayoutLMConfig
Author: arnaudstiegler
Pull request: yes

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Related to #19303 

Remove LayoutLMConfig dependence on BertConfig.

The `__init__` from `LayoutLMConfig` diverges from `BertConfig` in the following way:
- `max_2d_position_embeddings` is an argument specific to LayoutLM
For that reason, I didn't add any `# Copied from ...`

Other change:
Previously, the arguments `position_embedding_type`, `use_cache`, `classifier_dropout` were not part of the `__init__` function for LayoutLM and were always set to the BertConfig default (through the `super().__init___()` call). Because there is no inheritance anymore, I added back those arguments to `__init__`





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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Welcome to the repo, Arnaud :)

Pinging @sgugger for review