* Ensure that add_prefix_space is propagated to backend_tokenizer.pre_tokenizer
in PreTrainedTokenizerFast, rather than relying on subclasses to take care of this.
* Simplify setting self.add_prefix_space, ensure pre_tok exists
* Wrap in try-except to catch 'Custom PreTokenizer cannot be serialized'
862d1a346a/bindings/python/src/pre_tokenizers.rs (L672) produces the Exception. They're triggered by the roformer tests, as the RoFormerTokenizerFast uses a custom PreTokenizer.
* Propagate add_prefix_space in T5TokenizerFast to superclass
32e0db8a69