llama : add StarCoder2 support (#5795)

* Add support for starcoder2

* handle rope type

* skip rope freq and rotary embeddings from being serialized

* resolve comments

* Update llama.cpp

* remove redundant changes

* handle `rope-theta`

* llama : change starcoder2 rope type

* address comment

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