Describe the bug
On the first attempt to call evaluate() in the train_loop() function, the DDPMPipeline fails due to an incompatible output form the model.
This raises an AttributeError: 'dict' object has no attribute 'shape'

Reproduction
Currently encountered when running the collab notebook unaltered.
Logs
Logs included in bug description.
System Info
Google Collab Notebook found at https://colab.research.google.com/github/huggingface/notebooks/blob/main/diffusers/training_example.ipynb