add sigmoid betas (#777)

* add sigmoid betas

* convert to torch

* add comment on source

src/diffusers/schedulers/scheduling_ddpm.py

@@ -133,6 +133,10 @@ class DDPMScheduler(SchedulerMixin, ConfigMixin):
         elif beta_schedule == "squaredcos_cap_v2":
             # Glide cosine schedule
             self.betas = betas_for_alpha_bar(num_train_timesteps)
+        elif beta_schedule == "sigmoid":
+            # GeoDiff sigmoid schedule
+            betas = torch.linspace(-6, 6, num_train_timesteps)
+            self.betas = torch.sigmoid(betas) * (beta_end - beta_start) + beta_start
         else:
             raise NotImplementedError(f"{beta_schedule} does is not implemented for {self.__class__}")