quantize : add '--keep-split' to quantize model into shards (#6688)

* Implement '--keep-split' to quantize model into several shards

* Add test script

* Update examples/quantize/quantize.cpp

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

* Split model correctly even if tensor id is out-of-order

* Update llama_model_quantize_params

* Fix preci failures

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Co-authored-by: z5269887 <z5269887@unsw.edu.au>
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>