llama : fix segfault from unknown model arch name (#5820)

llama : fix segfault from unknown model arch name (#5820)

* llama : fix segfault from unknown model arch name

* llama : make all LLM maps const

This also requires using `std::map::at` instead of its `operator[]`
which does not exist for const maps.

* llama : name LLM_ARCH_UNKNOWN to "(unknown)"

This avoids errors from `std::map::at` when
getting the general name of the model architecture.
Using "(unknown)" instead of an empty string as per suggestion
https://github.com/ggerganov/llama.cpp/pull/5820#issuecomment-1973735284

* llama : remove redundant inner const for LLM_TENSOR_NAMES

The extra const won't do anything here as const maps
return const references to values.

Co-authored-by: Jared Van Bortel <redacted>
* llama : remove redundant nullptr check in llm_arch_from_string

Since LLM_ARCH_NAMES is a const map, no spurious elements
with a NULL name are inserted anymore, so this check is dead code.

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Co-authored-by: Jared Van Bortel <redacted>