How can I provide a certain proposal width to my sampler and not let it change during the entire sampling?

I see that PyMC3 executes the following code to scale the proposal width. I don’t want this to happen and I would like to provide my own standard deviation for the normal distribution proposal. Is this possible?

```
if acc_rate < 0.001:
# reduce by 90 percent
scale *= 0.1
elif acc_rate < 0.05:
# reduce by 50 percent
scale *= 0.5
elif acc_rate < 0.2:
# reduce by ten percent
scale *= 0.9
elif acc_rate > 0.95:
# increase by factor of ten
scale *= 10.0
elif acc_rate > 0.75:
# increase by double
scale *= 2.0
elif acc_rate > 0.5:
# increase by ten percent
scale *= 1.1
```

In addition, I believe that the PyMC3 executes the above code only during the burn-in period. If yes, then after burn-in period what scaling factor does the sampler use? Is it the scaling value used in the last step of the burn-in period?