Raise error in from_id if the session has no backend

[kt474]

Summary

Details and comments

Fixes #1959

[ElePT]

Based off the PR removing the Optional type hint from Session, it seems clear that sessions should always have backends right? However if a user creates a session and doesn't send any jobs the backend doesn't get set. The error seems fine but I don't think this behavior is very intuitive. I wonder if there is a way to set the backend in the session from the moment it's created without requiring jobs?

[kt474]

Based off the PR removing the Optional type hint from Session, it seems clear that sessions should always have backends right? However if a user creates a session and doesn't send any jobs the backend doesn't get set. The error seems fine but I don't think this behavior is very intuitive. I wonder if there is a way to set the backend in the session from the moment it's created without requiring jobs?

Yeah I agree it's not intuitive - it's been a long ongoing discussion see ntc 5063 for more context