Learnable modules in ddgm example

In the ddgm example shouldn't the line:

```self.p_dnns = p_dnns```

be:

```self.p_dnns = nn.ModuleList( p_dnns)```

My understanding is that by not placing them in a ```ModuleList``` the ```p_dnns``` networks are not added in the learned parameters and are left unoptimized.

Thanks for an awesome resource by the way!
