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fix(LTXEulerAncestralRFScheduler): validate monotonicity of explicit sigma schedules #13466

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fix(LTXEulerAncestralRFScheduler): validate monotonicity of explicit sigma schedules #13466

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8 changes: 8 additions & 0 deletions src/diffusers/schedulers/scheduling_ltx_euler_ancestral_rf.py
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Original file line number Diff line number Diff line change
Expand Up @@ -233,6 +233,14 @@ def set_timesteps(
if sigmas_tensor.ndim != 1:
raise ValueError(f"`sigmas` must be a 1D tensor, got shape {tuple(sigmas_tensor.shape)}.")

if len(sigmas_tensor) > 1 and not torch.all(sigmas_tensor[:-1] >= sigmas_tensor[1:]):
raise ValueError(
"`sigmas` must be monotonically non-increasing (each sigma must be >= the next). "
"A non-monotone sigma schedule violates the CONST parametrization invariant of "
"LTXEulerAncestralRFScheduler: `step()` computes `sigma_down` from adjacent pairs, "
"and a schedule increase causes `alpha_down < 0`, which corrupts denoising silently."
)

if sigmas_tensor[-1].abs().item() > 1e-6:
logger.warning(
"The last sigma in the schedule is not zero (%.6f). "
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