Struct DecodeDims 

pub struct DecodeDims {
    pub n_layers: usize,
    pub n_kv_heads: usize,
    pub head_dim: usize,
    pub hidden: usize,
    pub vocab: usize,
    pub rope_width: usize,
    pub embed_scale: Option<f64>,
}

The model-scalar geometry the shared build path needs.

Deliberately not a description of the KV state — that is fuel_inference::decode_state_spec’s job (moved there in the fuel-core dissolution; not an intra-doc link, since fuel-core cannot depend on fuel-inference to resolve one), and conflating the two is the GAP-166 mistake. These are the dimensions of the graph this path builds.

Fields

n_layers: usize

n_kv_heads: usize

head_dim: usize

hidden: usize

Model width (dim / hidden_size).

vocab: usize

rope_width: usize

Width of the RoPE cos/sin tables. Equal to head_dim for a full-rotary family (Llama, Qwen2); a partial-rotary family (Glm4) passes its rotary_dim. Carried as a value rather than assumed, because that one usize is the whole of Glm4’s divergence from this path.

embed_scale: Option<f64>

Multiplier applied to the token embeddings before the layer loop. None = no scaling, and None emits no graph node at all — a Some(1.0) would still cost a mul_scalar, which is why this is an Option rather than a plain f64 defaulting to 1.

Gemma-family models pass Some(sqrt(hidden_size)); everyone else None.

⚠️ It is the SEAM that applies this, deliberately, and the ordering is the reason. The scale must land BEFORE the activation dtype cast: prefill scales in f32, so scaling after a cast would round in bf16 on a bf16 cache and diverge — invisible on an f32 gate. A hook would let each family put it wherever, making that hazard per-family and silent; a value applied at one fixed point makes the ordering unforgettable.

It is a scalar rather than a dimension, which the struct’s name does not quite cover — stated here rather than left to overreach quietly.

Trait Implementations

impl Clone for DecodeDims

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for DecodeDims

impl Debug for DecodeDims

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl PartialEq for DecodeDims

fn eq(&self, other: &Self) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for DecodeDims