DDPMScheduler allows num_inference_steps=0 without validation (inconsistent with DDIMScheduler)

Describe the bug

Bug description

The DDPMScheduler.set_timesteps method does not validate the value of num_inference_steps.

Passing num_inference_steps=0 does not raise an error and can lead to invalid internal state or unexpected behavior.

This is inconsistent with DDIMScheduler, which already validates this case and raises a ValueError.

I plan to submit a PR to fix this by adding proper validation and tests.

Reproduction

Reproduction

from diffusers import DDPMScheduler

scheduler = DDPMScheduler()
scheduler.set_timesteps(num_inference_steps=0)

Current behavior

No error is raised.

Expected behavior

ValueError: num_inference_steps must be > 0

Logs

No logs - issue is silent (no exception raised).

System Info

• Diffusers version: 0.38.0.dev0

• Platform: Windows-11

• Python version: 3.13.9

• PyTorch version (GPU?): 2.11.0+cpu (False)

• Huggingface_hub version: 1.8.0

• Safetensors version: 0.7.0

• Accelerator: NVIDIA GeForce RTX 2050 (4GB)

• Using GPU in script?: No

• Using distributed or parallel set-up in script?: No

Who can help?

@yiyixuxu

Start with DDPMScheduler.set_timesteps and compare its handling of num_inference_steps with DDIMScheduler. Reproduce the zero-step case, then add validation and a regression test confirming that zero raises the expected ValueError. Done means the invalid input is rejected consistently with DDIMScheduler.