UniPC with FlowMatch fails with index out-of-bounds

[vladmandic]

Describe the bug

If using new variant of UniPCMultistepScheduler introduced via #9982 it fails with index out-of-bounds on last step
(DPM is fine, I haven't checked other supported schedulers, but DEIS and SA are likely affected as well)

Reproduction

import torch
import diffusers

repo_id = 'Efficient-Large-Model/Sana_1600M_1024px_BF16_diffusers'
cache_dir = '/mnt/models/Diffusers'
prompt = 'sports cars parked in the city with people watching'
negative = ''
dtype = torch.bfloat16
device = torch.device('cuda')

pipe = diffusers.SanaPipeline.from_pretrained(repo_id, cache_dir=cache_dir, variant='bf16', torch_dtype=dtype)
pipe.scheduler = diffusers.UniPCMultistepScheduler(use_flow_sigmas=True, prediction_type='flow_prediction')
pipe = pipe.to(device=device, dtype=dtype)
result = pipe(
 prompt = prompt,
 negative_prompt = negative,
)
image = result.images[0]
image.save('/tmp/sana.png')

Logs

File "/home/vlado/dev/sdnext/venv/lib/python3.12/site-packages/diffusers/pipelines/sana/pipeline_sana.py", line 819, in __call__
 latents = self.scheduler.step(noise_pred, t, latents, **extra_step_kwargs, return_dict=False)[0]
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/home/vlado/dev/sdnext/venv/lib/python3.12/site-packages/diffusers/schedulers/scheduling_unipc_multistep.py", line 999, in step
 prev_sample = self.multistep_uni_p_bh_update(
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/home/vlado/dev/sdnext/venv/lib/python3.12/site-packages/diffusers/schedulers/scheduling_unipc_multistep.py", line 688, in multistep_uni_p_bh_update
 sigma_t, sigma_s0 = self.sigmas[self.step_index + 1], self.sigmas[self.step_index]
 ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
IndexError: index 20 is out of bounds for dimension 0 with size 20

System Info

diffusers==0.32.dev

Who can help?

@hlky @a-r-r-o-w @lawrence-cj