Function lower_forward_streamed 

pub fn lower_forward_streamed(
    spec: &ModelSpec,
    storage: Storage<'_>,
    target: &SpatialTarget,
    fill: impl Fn(&str) -> String,
) -> Result<StreamedModel, StreamedReject>

Lower spec as one program per layer kind plus the head, and bind every layer to its kind.

fill turns a generated program into the source that is actually lowered (the weights’ fills, when a caller has them); layer programs are lowered through it so a kind program carries the fills of its representative layer.