Make LayoutLM tokenizers independent from BertTokenizer
Author: arnaudstiegler
Pull request: yes

# What does this PR do?

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Decoupling `LayoutLMTokenizer` and `LayoutLMTokenizerFast` from `BertTokenizer`.

Since only a few class constants change between Bert and LayoutLM, there's a copy flag for every single method in the class.

I wonder whether having prefixes for the class constants could help reducing the amount of code, for instance:
`VOCAB_FILES_NAMES` -> `BERT_VOCAB_FILES_NAMES` 
so that we could simply use: `# Copied from transformers.models.bert.tokenization_bert_fast.BertTokenizerFast with Bert->LayoutLM` on the entire tokenizer class though I suspect there are good reasons not to do that.

Related to #19303 


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

@sgugger The PR is ready for review :)