Checkpoints cannot be loaded in non-pl env
## 🚀 Feature
Add an option to save only state_dict for ModelCheckpoint callbacks
🐛 Bug
PL checkpoints cannot be loaded in non-pl envs
Motivation
To be able to move trained models and weights into pytorch only environments
Additional context
Currently  when you do torch.load() on a pl generated checkpoint in an environment without pl, there is a pickling error. For my current use case I have to load the checkpoints in my training environment and save them again with only state_dict for the weights.
See reply below for more info