UnCLIPScheduler.set_timesteps crashes (ZeroDivisionError) for 1 step and silently returns an empty schedule for 0 steps

[kyo-zzz]

Describe the bug

UnCLIPScheduler.set_timesteps breaks for fewer than 2 inference steps:

• num_inference_steps == 1 crashes with a bare ZeroDivisionError;
• num_inference_steps == 0 silently returns an empty timestep schedule.

Root cause: the scheduler uses the karlo-style inclusive-endpoint step ratio (num_train_timesteps - 1) / (num_inference_steps - 1) (see set_timesteps in scheduling_unclip.py). The docstring does not state a minimum, and UnCLIPPipeline forwards the user's value straight through, so the low value surfaces as an opaque crash instead of a clear error.

Since the schedule interpolates the two endpoints of the training range, num_inference_steps == 1 has no well-defined interval; validating up front (minimum 2 steps, like MiniMaxH3Scheduler already does for the same situation) seems more appropriate than defining n=1 numerics.

I found this while sweeping schedulers for low-step edge cases in an agentic coding session (root-cause analysis developed with AI assistance; I verified the reproduction and the code path locally). I have already opened a fix: #14574.

Reproduction

from diffusers import UnCLIPScheduler

scheduler = UnCLIPScheduler()
scheduler.set_timesteps(1)   # ZeroDivisionError: division by zero

and

from diffusers import UnCLIPScheduler

scheduler = UnCLIPScheduler()
scheduler.set_timesteps(0)   # no error; scheduler.timesteps is empty
print(scheduler.timesteps)   # tensor([], dtype=torch.int64)

Logs

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "...\src\diffusers\schedulers\scheduling_unclip.py", line 191, in set_timesteps
    step_ratio = (self.config.num_train_timesteps - 1) / (self.num_inference_steps - 1)
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ZeroDivisionError: division by zero

System Info

• 🤗 Diffusers version: 0.40.0.dev0
• Platform: Windows-10-10.0.26200-SP0
• Running on Google Colab?: No
• Python version: 3.10.11
• PyTorch version (GPU?): 2.13.0+cpu (False)
• Huggingface_hub version: 1.28.0
• Transformers version: not installed
• Accelerate version: not installed
• PEFT version: not installed
• Safetensors version: 0.8.0
• xFormers version: not installed
• Accelerator: NVIDIA GeForce RTX 4060 Laptop GPU, 8188 MiB
• Using GPU in script?: No
• Using distributed or parallel set-up in script?: No

Who can help?

@yiyixuxu