Block-dispatch structs for Emily.Fast.* fused kernels.
Each Emily.Fast.* helper emits an Nx.block/4 node carrying one
of these structs. Emily.Backend block/4 pattern-matches on the
struct to call the matching mx::fast::* NIF; non-Emily backends
fall through to the default fun supplied by Nx.block/4, which
runs the composed-defn fallback baked into each helper.
Static configuration (eps, dims, scale, causal, …) is carried as
struct fields. Runtime tensors travel in the Nx.block/4 args list.