DDIMInverseScheduler.step: Incorrect Previous Timestep Calculation

[stop1one]

Describe the bug

There is a bug in DDIMInverseScheduler.step related to how the previous timestep is computed.
The inverse scheduler should move from timestep $t+1$ to $t$, but currently, the code is mistakenly computing $t-1$ instead of $t+1$.
https://github.com/huggingface/diffusers/blob/89e4d6219805975bd7d253a267e1951badc9f1c0/src/diffusers/schedulers/scheduling_ddim_inverse.py#L327C1-L331C10

This should be corrected to ensure the inverse process aligns properly with the intended scheduling.

# 1. get previous step value (=t+1)
prev_timestep = min(
    timestep + self.config.num_train_timesteps // self.num_inference_steps, 
    self.config.num_train_timesteps - 1
)

For comparison, DDIMScheduler correctly calculates $t-1$, as expected in a forward process:
https://github.com/huggingface/diffusers/blob/89e4d6219805975bd7d253a267e1951badc9f1c0/src/diffusers/schedulers/scheduling_ddim.py#L401C1-L402C95

Reproduction

This issue is straightforward to verify, so a reproduction snippet is omitted.

Logs

System Info

diffusers==0.32.2

Who can help?

@yiyixuxu