Function pool_instance_norm_means 

fn pool_instance_norm_means(proto: &mut ModelProto) -> Result<(), OnnxError>

Restate every ReduceMean over the spatial axes [2, 3, 4] (instance normalisation’s mean and variance) as an AveragePool of NORM_POOL³ windows, stride NORM_POOL, followed by the same ReduceMean over the window means. Mathematically the same mean (equal windows tiling the cube); numerically it keeps tract’s f32 sums short (module docs, Instance-norm statistics). Fails if the graph has no such reduction, so a re-exported graph cannot skip it silently.