feat(llama-cpp): expose split_mode option for multi-GPU placement (#9560)

Adds split_mode (alias sm) to the llama.cpp backend options allowlist,
accepting none|layer|row|tensor. The tensor value targets the experimental
backend-agnostic tensor parallelism from ggml-org/llama.cpp#19378 and
requires a llama.cpp build that includes that PR, FlashAttention enabled,
KV-cache quantization disabled, and a manually set context size.


Assisted-by: Claude:claude-opus-4-7

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>