Add spe_split_by_unicode_script arg
# What does this PR do ?

Expose the `spe_split_by_unicode_script` SentencePiece cmd line flag to the tokenizer generation script.

Arabic text contains diacritics that can influence the meaning of a word, therefore, they must be considered part of a character when generating a tokenizer vocabulary.
The `sentencepiece` library however does not consider diacritics as part of the word and generally considers it as a separate token altogether.
As a result, the diacritics are generally stripped as separate tokens from the word and the generated vocabulary does not properly represent the words in the dataset.

By adding this flag, we allow the SPE library to consider multiple unicode scripts as a single "piece".
See the comments on the flag [here](https://github.com/google/sentencepiece/blob/9f3ed99f5c68609fc94ac83a5c0aea9eb163f6d9/src/sentencepiece_model.proto#L135).

**Collection**: ASR

# Changelog 
- Expose the `spe_split_by_unicode_script` SentencePiece command line flag to the tokenizer generation script.

# Usage

```sh
python process_asr_text_tokenizer.py --manifest=/path/to/dataset_manifest.json \
        --data_root=tokenizers/datatset \
        --vocab_size=1024 \
        --tokenizer=spe \
        --spe_no_spe_split_by_unicode_script \
        --log
```

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