接着扩散模型简述训练扩散模型过程中用到的损失函数形式。完整的观察数据\(x\)的对数似然如下：\[\begin{aligned}\mathrm{log}\p(x)&\geq\mathbb{E}_{q_{\phi}(z_{1:T}|z_0)}\mathrm{log}\frac{p(z_T)\prod_{t=0}^{T-1}p_{\theta}(z_t|z_{t+1})}{\prod_{t=0}^{T-1}q_{\phi}