cmake : do not hide GGML options + rename option

[ggerganov]

Using set(GGML_XXX ON) before option(GGML_XXX OFF) will hide the options from configuration tools like ccmake. To fix this, we define GGML_XXX_DEFAULT and use it in option(GGML_XXX ${GGML_XXX_DEFAULT}). This allows ggml-based projects such as llama.cpp override the default option without hiding it from the config.

Also rename GGML_CUDA_USE_GRAPHS CMake option to GGML_CUDA_GRAPHS for consistency.

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