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fix: deduplicate timesteps in DPMSolverMultistepScheduler to prevent IndexError #13239

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fix: deduplicate timesteps in DPMSolverMultistepScheduler to prevent IndexError #13239

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15 changes: 11 additions & 4 deletions src/diffusers/schedulers/scheduling_dpmsolver_multistep.py
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Original file line number Diff line number Diff line change
Expand Up @@ -442,15 +442,11 @@ def set_timesteps(
sigmas = np.flip(sigmas).copy()
sigmas = self._convert_to_karras(in_sigmas=sigmas, num_inference_steps=num_inference_steps)
timesteps = np.array([self._sigma_to_t(sigma, log_sigmas) for sigma in sigmas])
if self.config.beta_schedule != "squaredcos_cap_v2":
timesteps = timesteps.round()
elif self.config.use_lu_lambdas:
lambdas = np.flip(log_sigmas.copy())
lambdas = self._convert_to_lu(in_lambdas=lambdas, num_inference_steps=num_inference_steps)
sigmas = np.exp(lambdas)
timesteps = np.array([self._sigma_to_t(sigma, log_sigmas) for sigma in sigmas])
if self.config.beta_schedule != "squaredcos_cap_v2":
timesteps = timesteps.round()
elif self.config.use_exponential_sigmas:
sigmas = np.flip(sigmas).copy()
sigmas = self._convert_to_exponential(in_sigmas=sigmas, num_inference_steps=num_inference_steps)
Expand All @@ -476,6 +472,17 @@ def set_timesteps(
f"`final_sigmas_type` must be one of 'zero', or 'sigma_min', but got {self.config.final_sigmas_type}"
)

# When sigma-to-timestep mapping produces duplicates after rounding to integer
# (e.g. cosine schedule with karras/lu sigmas where many sigmas map to nearly the same timestep),
# deduplicate while preserving order to prevent index out-of-bounds errors in multistep solvers.
# This mirrors the same fix in DPMSolverMultistepInverseScheduler.
timesteps = np.round(timesteps).astype(np.int64)
_, unique_indices = np.unique(timesteps, return_index=True)
unique_indices = np.sort(unique_indices)
if len(unique_indices) < len(timesteps):
timesteps = timesteps[unique_indices]
sigmas = sigmas[unique_indices]

sigmas = np.concatenate([sigmas, [sigma_last]]).astype(np.float32)

self.sigmas = torch.from_numpy(sigmas)
Expand Down