Fix regression #6840

The `tune` attribute is reset by the sampling iterator.

Closes #6840

Author: michaelosthege

pymc/step_methods/metropolis.py

@@ -492,7 +492,6 @@ def __init__(self, vars, order="random", transit_p=0.8, model=None):
 
     def reset_tuning(self):
         # There are no tuning parameters in this step method.
-        self.tune = False
         return
 
     def astep(self, apoint: RaveledVars, *args) -> Tuple[RaveledVars, StatsType]:
@@ -620,7 +619,6 @@ def __init__(self, vars, proposal="uniform", order="random", model=None):
 
     def reset_tuning(self):
         # There are no tuning parameters in this step method.
-        self.tune = False
         return
 
     def astep_unif(self, apoint: RaveledVars, *args) -> Tuple[RaveledVars, StatsType]: