Function xgboost_sys::XGBoosterBoostOneIter

pub unsafe extern "C" fn XGBoosterBoostOneIter(
    handle: BoosterHandle,
    dtrain: DMatrixHandle,
    grad: *mut f32,
    hess: *mut f32,
    len: bst_ulong
) -> c_int

\brief update the model, by directly specify gradient and second order gradient, this can be used to replace UpdateOneIter, to support customized loss function \param handle handle \param dtrain training data \param grad gradient statistics \param hess second order gradient statistics \param len length of grad/hess array \return 0 when success, -1 when failure happens