Function build_conv_fwd_graph 

pub fn build_conv_fwd_graph(
    dtype: DtypeTag,
    x_dims: &[i64],
    w_dims: &[i64],
    y_dims: &[i64],
    conv: &ConvDescParams,
    layout: TensorLayout,
    epilogue: EpilogueKind,
) -> OperationGraphSpec

Build the spec-side conv-fwd op graph, parameterised by dtype + dims. Independent of GpuRef so callers (tests, plan-cache lookup) can build it without owning device buffers.