Fixes in ddpm.py and main.py

[silky1708]

1. first move self.logvar to self.device, and then index using t (line 1030, ddpm.py)
   the current verison of the code gives the following error:

RuntimeError: indices should be either on cpu or on the same device as the indexed tensor (cpu)

2. remove extraneous argument outputs from on_train_epoch_end in main.py
   the current version of the code gives the following error:

TypeError: on_train_epoch_end() missing 1 required positional argument: 'outputs'