Questions about code in diffusion_transformer.py (q_pred)

[Ed-ivan]

Thanks for your excellent works and sharing your code !
while I have questions about the code:
log_probs = torch.cat(
[
log_add_exp(log_x_start[:,:-1,:]+log_cumprod_at, log_cumprod_bt),
log_add_exp(log_x_start[:,-1:,:]+log_1_min_cumprod_ct, log_cumprod_ct)
],
dim=1
)
why log_add_exp is used , looking forward your reply!

[Ed-ivan]

while I notice that in line 212 of diffusion_transformer.py,
log_qt=self.q_pred(log_x_t,t) ,which is called by q_posterior(self,log_x_start,log_x_t,t) .
I feel a little confused that why is log_x_t is used but not log_x_start

looking forward your reply!