llama : allow getting n_batch from llama_context in c api (#4540)
* allowed getting n_batch from llama_context in c api
* changed to use `uint32_t` instead of `int`
* changed to use `uint32_t` instead of `int` in `llama_n_ctx`
* Update llama.h
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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>