add scaled_linear schedule in PNDM and DDPM (#164)

src/diffusers/schedulers/scheduling_ddpm.py

@@ -65,6 +65,9 @@ class DDPMScheduler(SchedulerMixin, ConfigMixin):
             self.betas = np.asarray(trained_betas)
         elif beta_schedule == "linear":
             self.betas = np.linspace(beta_start, beta_end, num_train_timesteps, dtype=np.float32)
+        elif beta_schedule == "scaled_linear":
+            # this schedule is very specific to the latent diffusion model.
+            self.betas = np.linspace(beta_start**0.5, beta_end**0.5, num_train_timesteps, dtype=np.float32) ** 2
         elif beta_schedule == "squaredcos_cap_v2":
             # Glide cosine schedule
             self.betas = betas_for_alpha_bar(num_train_timesteps)

src/diffusers/schedulers/scheduling_pndm.py

@@ -60,6 +60,9 @@ class PNDMScheduler(SchedulerMixin, ConfigMixin):
 
         if beta_schedule == "linear":
             self.betas = np.linspace(beta_start, beta_end, num_train_timesteps, dtype=np.float32)
+        elif beta_schedule == "scaled_linear":
+            # this schedule is very specific to the latent diffusion model.
+            self.betas = np.linspace(beta_start**0.5, beta_end**0.5, num_train_timesteps, dtype=np.float32) ** 2
         elif beta_schedule == "squaredcos_cap_v2":
             # Glide cosine schedule
             self.betas = betas_for_alpha_bar(num_train_timesteps)