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

[sfluegel05]

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

[aditya0by0]

Lets try and assign this to co-pilot to check whether it can take upon minor issues.