fix LayoutLMv3TokenizerFast subword label after 'Ġ' token
LayoutLMv3TokenizerFast produces empty 'Ġ' token with `offset_mapping = (0, 0)`. 
Next token is wrongly assumed to also be beginning of word and isn't correctly assigned `pad_token_label`.
This may lead to misalignment of words and token representations.
Other BPE tokenizers might be affected

Add check for previous token if it had an empty `offset_mapping` (not including special tokens)
Remove copy check from LayoutLMv2TokenizerFast for `_batch_encode_plus` because it is not affected (uses WordPiece instead of BPE)
Modify test with text that produce 'Ġ' token.

Fixes issue: #19978

@NielsRogge 
@ArthurZucker 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_21695). All of your documentation changes will be reflected on that endpoint.<|||||>Also cc @amyeroberts <|||||>Hi @ArthurZucker, thanks for your investigations.

This PR fixes the problem for LayoutLMv3 but I expect the problem to exist on other models using Fast BPE tokenization, I will take a look when I can to list all impacted models that need a fix.<|||||>Thanks a lot for this fix, would you be able to take into account my comment such that we can merge it? 🙏 

Thanks!

Btw the same fix could then be applied to LayoutLMv2 and LayoutXLM<|||||>LayoutLMv2 uses WordPiece and not BPE. From what I saw its vocabulary does not contain empty token and thus cannot produce (0, 0) offset_mapping when encoding.
