llama : implement Unigram tokenizer needed by T5 and FLAN-T5 model families (#5763)

* llama : add T5 model architecture, tensors and model header parameters

* llama : add implementation of Unigram tokenizer with SentencePiece-like text normalization using precompiled charsmap

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Co-authored-by: Stanisław Szymczyk <sszymczy@gmail.com>