Function build_rnn_fwd_spec 

pub fn build_rnn_fwd_spec(
    dtype: DtypeTag,
    p: &RnnParams,
    layout: TensorLayout,
) -> OperationGraphSpec

Build a spec-side OperationGraphSpec for plan-cache keying. The actual RNN API does not use the v9 backend graph descriptor builder — but we keep a stable signature surface so the cache key machinery is shared.