Do not use class variables in runtime modules

[wwwillchen]

• It's better to do the type annotations inside __init__ instead of annotating at the class variable.
• Initializing class variables is sketchy, particularly with mutable objects.
• We were previously initializing class variables to mutable values on Runtime, which is technically not too bad because Runtime is a singleton object, but this is still a bad practice.

[richard-to]

Oh wow, that's a great catch. Can't believe I never noticed that.

[wwwillchen]

Oh wow, that's a great catch. Can't believe I never noticed that.

yeah, I was a little shocked and then I realized it's because dataclass purposely blurs the line between between class variables and instance variables (e.g. you define it as a class variable but you use it like an instance variable) so it's easy to get this mixed up :(