Question about NaN in DPMSolverMultistepInverseScheduler

[DekuLiuTesla]

Hi, everyone, I'm new to diffusers. I'm trying to use DPMSolverMultistepInverseScheduler for DDIM inversion. The applied config is:

dpmpp_2m_sde_karras_scheduler_inv = DPMSolverMultistepInverseScheduler(
    num_train_timesteps=1000,
    beta_start=0.00085,
    beta_end=0.012,
    algorithm_type="sde-dpmsolver++",
    use_karras_sigmas=True,
    steps_offset=1
)

And the DDIM inversion is realized through:

self.scheduler.set_timesteps(self.inv_config.steps)
timesteps = self.scheduler.timesteps
with torch.autocast(device_type=self.device, dtype=self.dtype):
    for i, t in enumerate(tqdm(timesteps)):
        noises = []
        x_index = torch.arange(len(x))
        batches = x_index.split(self.batch_size, dim = 0)
        for batch in batches:
            noise = self.pred_noise(
                x[batch], conds, timesteps[i], concat_conds=x[batch], batch_idx=batch)
            noises += [noise]
        noises = torch.cat(noises)
        
        x = self.scheduler.step(noises, t, x, generator=self.rng, return_dict=False)[0]

But NaN occurs in the first scheduler step. I dug into it and found it happens in dpm_solver_first_order_update:

diffusers/src/diffusers/schedulers/scheduling_dpmsolver_multistep_inverse.py

Lines 627 to 633 in 560fb5f

	sigma_t, sigma_s = self.sigmas[self.step_index + 1], self.sigmas[self.step_index]
	alpha_t, sigma_t = self._sigma_to_alpha_sigma_t(sigma_t)
	alpha_s, sigma_s = self._sigma_to_alpha_sigma_t(sigma_s)
	lambda_t = torch.log(alpha_t) - torch.log(sigma_t)
	lambda_s = torch.log(alpha_s) - torch.log(sigma_s)
	h = lambda_t - lambda_s

The self.sigmas is tensor([ 0.0292, 0.0462, 0.0710, 0.1065, 0.1563, 0.2249, 0.3178, 0.4417, 0.6050, 0.8176, 1.0911, 1.4396, 1.8795, 2.4300, 3.1132, 3.9548, 4.9844, 6.2356, 7.7471, 9.5622, 11.7303, 14.3068, 17.3539, 20.9411, 25.1461, 25.1461]). Its increasing order leads lambda_t to be smaller than lambda_s and therefore a negative h.
As a result, in

diffusers/src/diffusers/schedulers/scheduling_dpmsolver_multistep_inverse.py

Lines 638 to 644 in 560fb5f

	elif self.config.algorithm_type == "sde-dpmsolver++":
	assert noise is not None
	x_t = (
	(sigma_t / sigma_s * torch.exp(-h)) * sample
	+ (alpha_t * (1 - torch.exp(-2.0 * h))) * model_output
	+ sigma_t * torch.sqrt(1.0 - torch.exp(-2 * h)) * noise
	)

torch.sqrt(1.0 - torch.exp(-2 * h)) becomes NaN. But I noticed that in DPMSolverMultistepScheduler, the problem is avoided by applying flip:

diffusers/src/diffusers/schedulers/scheduling_dpmsolver_multistep.py

Lines 395 to 396 in 560fb5f

	if self.config.use_karras_sigmas:
	sigmas = np.flip(sigmas).copy()

I've searched for many usage examples, but I still can't figure out the stem of the problem. Is there anybody that can give a help?🙏