minor : add const qualifiers (#2853)

* made the methods const

# Conflicts:
#	examples/convert-llama2c-to-ggml/convert-llama2c-to-ggml.cpp

* made method const

* Update convert-llama2c-to-ggml.cpp

removed write_raw and write_u32

* llama2c : remove misleading const

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