Algorithm 2 Inference in T iterative refinement steps

1:${\mathbf{x}}_T\sim \mathcal{N}(0,\mathbf{I})$ 2:for  $t=T,\dots ,1$  do 3:    $\mathit{ϵ}\sim \mathcal{N}(\mathbf{0},\mathbf{I})$if$t>1$, else $\mathit{ϵ}=0$ 4:    ${\mathbf{x}}_{t-1}=\frac{1}{\sqrt{{\overline{\alpha }}_t}}\left({\mathbf{x}}_t-\frac{1-{\overline{\alpha }}_t}{\sqrt{1-{\overline{\alpha }}_t}}{\mathit{ϵ}}_{\theta }({\mathbf{x}}_t,\mathbf{z},t)\right)+\sqrt{1-{\overline{\alpha }}_t}\mathit{ϵ}$ 5:end for 6:return  ${\mathbf{x}}_0$