Add `jsonargparse` to main dependencies, only use sklearn if needed

We had two issues when installing chebai for inference only:
- missing `jsonargparse` caused some error in lightning
- missing `scikit-learn` causes issues with the pubchem dataset

Both dependencies are included in the `[dev]` extra. 

## Todo
- move `jsonargparse` to the regular dependencies
- only load `sklearn` when a PubChem dataset is actually instantiated