fix: (issue #32124) Exception raised when running `transformers/examples/flax/language-modeling/t5_tokenizer_model.py`.
Author: fshp971
Pull request: yes

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Fixes #32124 (issue)

The original [t5_tokenizer_model.py](https://github.com/huggingface/transformers/tree/main/examples/flax/language-modeling/t5_tokenizer_model.py) uses argument `add_prefix_space` when creating [tokenizers.pre_tokenizers.Metaspace](https://github.com/huggingface/tokenizers/blob/main/tokenizers/src/pre_tokenizers/metaspace.rs).
However, such an argument has been dropped and replaced with argument `prepend_scheme` since `tokenizers 0.19.1` (see this [commit](https://github.com/huggingface/tokenizers/commit/09069717e94e1a22010d340bb947d7e49ef2baa9)).
So I update the code to make it compatible with `tokenizers >= 0.19.1`.


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Comments:
cc @ArthurZucker