Removed duplicated _Overload* methods in variables

Prior to this change all variable classes (Variable, RefVariable and
ResourceVariable) had their own copy/pasted implementation of
_OverloadOperator and _OverloadAllOperators. This was unnecessary since
a single implementation in the base class would work just as well.

The change also fixes the __name__ and __module__ of the operator
wrapper functions.

PiperOrigin-RevId: 222963169