 \left\| \mathop{\mathbb{E}}_{\boldsymbol{x}\sim p_{u}(\boldsymbol{x})}(\boldsymbol{f}(\boldsymbol{x})) - \mathop{\mathbb{E}}_{\boldsymbol{z}_g\sim p_{z}(z)}(\boldsymbol{f}(G(\boldsymbol{z}_g))) \right\|_0^0 \