Replace AttributeDict in with dict in checkpoint
🚀 Feature
When we save hyperparameters to the checkpoint, we save them in a Lightning datastructure called AttributeDict.
We should replace this with a regular dict when saving the checkpoint.
Motivation
Allows loading checkpoints with torch.load in environments where Lightning is not installed.
Pitch
Convert AttributeDict to dict when saving, convert dict to AttributeDict when loading.
cc @Borda