clamp comment @sayakpaul

src/diffusers/schedulers/scheduling_ddpm.py

@@ -214,6 +214,8 @@ class DDPMScheduler(SchedulerMixin, ConfigMixin):
         # and sample from it to get previous sample
         # x_{t-1} ~ N(pred_prev_sample, variance) == add variance to pred_sample
         variance = (1 - alpha_prod_t_prev) / (1 - alpha_prod_t) * current_beta_t
+
+        # we always take the log of variance, so clamp it to ensure it's not 0
         variance = torch.clamp(variance, min=1e-20)
 
         if variance_type is None: