Apple MLX backend (EMLX + EMLXAxon).
Loads the same HuggingFace repositories through Bumblebee but onto the
EMLX.Backend Nx backend, rewriting LLM graphs with EMLXAxon Metal
shaders when that optional dependency is present. Returns
{:error, :emlx_unavailable} when the platform or dependencies cannot run
MLX so ModelManager can fall back to EXLA.