Remove `model_connector.py`

[daniellepintz]

Proposed refactoring or deprecation

model_connector.py has only one function copy_trainer_model_properties which is used in one place, data_connector.py. Let's move copy_trainer_model_properties directly to data_connector.py.

Motivation

We are auditing the connectors in the Architecture doc (https://docs.google.com/document/d/1xHU7-iQSpp9KJTjI3As2EM0mfNHHr37WZYpDpwLkivA/edit?pli=1#heading=h.rjokms600dhn). We can simplify things by removingmodel_connector.py and moving the function to data_connector.py.

[tchaton]

Dear @daniellepintz,

I am good with this. @awaelchli @carmocca @Borda Good with you ?

Best,
T.C

[awaelchli]

Yes fine with me. On top of that the code in there can probably be simplified even more than it is :)

[daniellepintz]

Yup, I'm doing the moving first in one PR and then we can do simplification PRs afterwards, since if we did them in the same PR it would be hard to review