Type rv_op based on behaviour rather than type

This is a bit like using a Protocol class, we say that whatever
is passed to rv_op, it should return a TensorVariable.

pymc/distributions/distribution.py

@@ -443,8 +443,8 @@ def change_symbolic_rv_size(op: SymbolicRandomVariable, rv, new_size, expand) ->
 class Distribution(metaclass=DistributionMeta):
     """Statistical distribution"""
 
-    rv_op: RandomVariable | SymbolicRandomVariable = None
-    rv_type: MetaType = None
+    rv_op: Callable[..., TensorVariable] | None = None
+    rv_type: MetaType | None = None
 
     def __new__(
         cls,