expo_decay() can be used to increase or decrease a function exponentially
over iterations. This can be used to dynamically set parameters for
acquisition functions as iterations of Bayesian optimization proceed.
expo_decay(iter, start_val, limit_val, slope = 1/5)
An integer for the current iteration number.
The number returned for the first iteration.
The number that the process converges to over iterations.
A coefficient for the exponent to control the rate of decay. The sign of the slope controls the direction of decay.
A single numeric value.
Note that, when used with the acquisition functions in
tune(), a wrapper
would be required since only the first argument would be evaluated during