allow loading ddpm models into ddim (huggingface#1932)

Author: patrickvonplaten

src/diffusers/pipelines/ddim/pipeline_ddim.py

@@ -16,6 +16,7 @@
 
 import torch
 
+from ...schedulers import DDIMScheduler
 from ...utils import deprecate, randn_tensor
 from ..pipeline_utils import DiffusionPipeline, ImagePipelineOutput
 
@@ -34,6 +35,10 @@ class DDIMPipeline(DiffusionPipeline):
 
     def __init__(self, unet, scheduler):
         super().__init__()
+
+        # make sure scheduler can always be converted to DDIM
+        scheduler = DDIMScheduler.from_config(scheduler.config)
+
         self.register_modules(unet=unet, scheduler=scheduler)
 
     @torch.no_grad()