clone cfg to make it importable from other files; remove pl dependency on load

[alexstoken]

Hi Georg,

I'm trying to integrate this work into https://github.com/gmberton/image-matching-models (IMM) and found a few hiccups.

1. importing the cfg doesnt take up the changes from within the file unless the cfg is cloned at the top
2. pytorch lightning is required to load due to its one function call inside utils/misc.py and that the model checkpoints are saved as lightning checkpoints. Fix for the former is in this PR, for the latter I saved out only the state dict (removing the optimizer, etc from the .ckpt file you share) and saved that out at https://drive.google.com/drive/folders/1VDYMw-vZ9aE_0cNOPNEz08DeLfQPapKO

If these look ok to you then we could add this to IMM. Thanks and thanks for all your great work on this project.