[2064]: Add Karras to DPMSolverMultistepScheduler

src/diffusers/schedulers/scheduling_dpmsolver_multistep.py

@@ -114,7 +114,10 @@ class DPMSolverMultistepScheduler(SchedulerMixin, ConfigMixin):
         lower_order_final (`bool`, default `True`):
             whether to use lower-order solvers in the final steps. Only valid for < 15 inference steps. We empirically
             find this trick can stabilize the sampling of DPM-Solver for steps < 15, especially for steps <= 10.
-
+        use_karras_sigmas (`bool`, *optional*, defaults to `False`):
+             This parameter controls whether to use Karras sigmas (Karras et al. (2022) scheme) for step sizes in the
+             noise schedule during the sampling process. If True, the sigmas will be determined according to a sequence
+             of noise levels {σi} as defined in Equation (5) of the paper https://arxiv.org/pdf/2206.00364.pdf.
     """
 
     _compatibles = [e.name for e in KarrasDiffusionSchedulers]
@@ -136,6 +139,7 @@ def __init__(
         algorithm_type: str = "dpmsolver++",
         solver_type: str = "midpoint",
         lower_order_final: bool = True,
+        use_karras_sigmas: Optional[bool] = False,
     ):
         if trained_betas is not None:
             self.betas = torch.tensor(trained_betas, dtype=torch.float32)
@@ -181,6 +185,7 @@ def __init__(
         self.timesteps = torch.from_numpy(timesteps)
         self.model_outputs = [None] * solver_order
         self.lower_order_nums = 0
+        self.use_karras_sigmas = use_karras_sigmas
 
     def set_timesteps(self, num_inference_steps: int, device: Union[str, torch.device] = None):
         """
@@ -199,6 +204,12 @@ def set_timesteps(self, num_inference_steps: int, device: Union[str, torch.devic
             .copy()
             .astype(np.int64)
         )
+        if self.use_karras_sigmas:
+            sigmas = np.array(((1 - self.alphas_cumprod) / self.alphas_cumprod) ** 0.5)
+            log_sigmas = np.log(sigmas)
+            sigmas = self._convert_to_karras(in_sigmas=sigmas)
+            timesteps = np.array([self._sigma_to_t(sigma, log_sigmas) for sigma in sigmas]).round()
+            timesteps = np.flip(timesteps).copy().astype(np.int64)
         self.timesteps = torch.from_numpy(timesteps).to(device)
         self.model_outputs = [
             None,
@@ -217,6 +228,44 @@ def _threshold_sample(self, sample: torch.FloatTensor) -> torch.FloatTensor:
         )
         return sample.clamp(-dynamic_max_val, dynamic_max_val) / dynamic_max_val
 
+    # copied from diffusers.schedulers.scheduling_euler_discrete._sigma_to_t
+    def _sigma_to_t(self, sigma, log_sigmas):
+        # get log sigma
+        log_sigma = np.log(sigma)
+
+        # get distribution
+        dists = log_sigma - log_sigmas[:, np.newaxis]
+
+        # get sigmas range
+        low_idx = np.cumsum((dists >= 0), axis=0).argmax(axis=0).clip(max=log_sigmas.shape[0] - 2)
+        high_idx = low_idx + 1
+
+        low = log_sigmas[low_idx]
+        high = log_sigmas[high_idx]
+
+        # interpolate sigmas
+        w = (low - log_sigma) / (low - high)
+        w = np.clip(w, 0, 1)
+
+        # transform interpolation to time range
+        t = (1 - w) * low_idx + w * high_idx
+        t = t.reshape(sigma.shape)
+        return t
+
+    # copied from diffusers.schedulers.scheduling_euler_discrete._convert_to_karras
+    def _convert_to_karras(self, in_sigmas: torch.FloatTensor) -> torch.FloatTensor:
+        """Constructs the noise schedule of Karras et al. (2022)."""
+
+        sigma_min: float = in_sigmas[-1].item()
+        sigma_max: float = in_sigmas[0].item()
+
+        rho = 7.0  # 7.0 is the value used in the paper
+        ramp = np.linspace(0, 1, self.num_inference_steps)
+        min_inv_rho = sigma_min ** (1 / rho)
+        max_inv_rho = sigma_max ** (1 / rho)
+        sigmas = (max_inv_rho + ramp * (min_inv_rho - max_inv_rho)) ** rho
+        return sigmas
+
     def convert_model_output(
         self, model_output: torch.FloatTensor, timestep: int, sample: torch.FloatTensor
     ) -> torch.FloatTensor: