Stateless layer primitives: RMSNorm, SwiGLU.
rms_norm/3 delegates to EMLX.Fast.rms_norm (single fused Metal shader).
RoPE is no longer computed here — EMLX.Fast.rope/6 is called directly in
Attention.forward/10 after projecting and transposing to {B, N, T, D}.
Summary
Functions
Root-mean-square layer normalisation via mlx::fast::rms_norm.
SwiGLU activation: silu(gate) * up. gate and up must have the same shape.