fix LayoutLMv3TokenizerFast subword label after 'Ġ' token (#21695)
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`.
Modify test with text that produce 'Ġ' token.
Remove copy check from LayoutLMv2TokenizerFast for `_batch_encode_plus`.
solves issue: #19978