[ `PreTrainedTokenizerFast`] Keep properties from fast tokenizer
# What does this PR do?
Start a potential fix to #24179, ended up being releated to #24441, calling modifies the values of the underlying tokenizer, but never changes anything on the surface so will probably add some kind of warning in the documentation. 

TLDR; adds the possibility  of initialize a `PreTrainedTokenizerFast` from a `tokenizer.Tokenizer`, keeping the `padding` and `truncation` informations. 


_The documentation is not available anymore as the PR was closed or merged._<|||||>Yes, currently everything is ignored