Allow DDPM scheduler to use model's predicated variance (#132)

* Extented the ability of ddpm scheduler
to utilize model that also predict the variance.

* Update src/diffusers/schedulers/scheduling_ddpm.py

Co-authored-by: Anton Lozhkov <aglozhkov@gmail.com>

src/diffusers/schedulers/scheduling_ddpm.py

@@ -82,6 +82,8 @@ class DDPMScheduler(SchedulerMixin, ConfigMixin):
         self.tensor_format = tensor_format
         self.set_format(tensor_format=tensor_format)
 
+        self.variance_type = variance_type
+
     def set_timesteps(self, num_inference_steps):
         num_inference_steps = min(self.config.num_train_timesteps, num_inference_steps)
         self.num_inference_steps = num_inference_steps
@@ -90,7 +92,7 @@ class DDPMScheduler(SchedulerMixin, ConfigMixin):
         )[::-1].copy()
         self.set_format(tensor_format=self.tensor_format)
 
-    def _get_variance(self, t, variance_type=None):
+    def _get_variance(self, t, predicted_variance=None, variance_type=None):
         alpha_prod_t = self.alphas_cumprod[t]
         alpha_prod_t_prev = self.alphas_cumprod[t - 1] if t > 0 else self.one
 
@@ -113,6 +115,13 @@ class DDPMScheduler(SchedulerMixin, ConfigMixin):
         elif variance_type == "fixed_large_log":
             # Glide max_log
             variance = self.log(self.betas[t])
+        elif variance_type == "learned":
+            return predicted_variance
+        elif variance_type == "learned_range":
+            min_log = variance
+            max_log = self.betas[t]
+            frac = (predicted_variance + 1) / 2
+            variance = frac * max_log + (1 - frac) * min_log
 
         return variance
 
@@ -125,6 +134,12 @@ class DDPMScheduler(SchedulerMixin, ConfigMixin):
         generator=None,
     ):
         t = timestep
+        
+        if model_output.shape[1] == sample.shape[1] * 2 and self.variance_type in ["learned", "learned_range"]:
+            model_output, predicted_variance = torch.split(model_output, sample.shape[1], dim=1)
+        else:
+            predicted_variance = None
+
         # 1. compute alphas, betas
         alpha_prod_t = self.alphas_cumprod[t]
         alpha_prod_t_prev = self.alphas_cumprod[t - 1] if t > 0 else self.one
@@ -155,7 +170,7 @@ class DDPMScheduler(SchedulerMixin, ConfigMixin):
         variance = 0
         if t > 0:
             noise = self.randn_like(model_output, generator=generator)
-            variance = (self._get_variance(t) ** 0.5) * noise
+            variance = (self._get_variance(t, predicted_variance=predicted_variance) ** 0.5) * noise
 
         pred_prev_sample = pred_prev_sample + variance