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>