Fix DDIMScheduler.step() prev_timestep for non-leading spacing

[Mr-Neutr0n]

Summary

• Fix DDIMScheduler.step() to correctly compute prev_timestep when timestep_spacing is set to linspace or trailing
• The previous implementation used a hard-coded formula (timestep - num_train_timesteps // num_inference_steps) that only produces correct values for timestep_spacing="leading"
• Replace with a previous_timestep() method that looks up the next timestep from self.timesteps, consistent with the approach already used in DDPMScheduler

Fixes #12633

Test plan

• Verify DDIM sampling produces correct results with timestep_spacing="linspace"
• Verify DDIM sampling produces correct results with timestep_spacing="trailing"
• Verify no regression with default timestep_spacing="leading"