Add helper for `special.kn` that just calls `special.kv` under the hood

[SimonEnsemble]

After

I wish for

pytensor.tensor.math.kn(0, 1.0)

to compute the modified Bessel function of the second kind, of order zero, at a generic point on its domain (e.g. 1.0 here).

Context for the issue:

I am unable to use Scipy's implementation scipy.special.kn (link) of the modified Bessel function of the second kind of integer order n (n=0 is what I need) in PyMC since it does not handle autodiff.

I see there are many Bessel functions implemented already in pytensor, but the modified Bessel function of the second kind, of order zero, is not. I am kindly requesting an implementation of this for my project modeling the spread of chemical plumes (link) to search for the source of a chemical plume using Bayesian inference in PyMC.

[SimonEnsemble]

the pytensor.tensor.math docs here were missing the integer order one. but I see the real order one work with zero, so I'm in business, I think.

# Define the expression
expr = pytensor.tensor.kv(0, 0.1)

# Compile the expression into a function
f = pytensor.function([], expr)

# Call the function to get the numerical value
result = f()
result

matches

scipy.special.kn(0, 0.1)

[ricardoV94]

@SimonEnsemble we should add a helper for kn that just calls kv so future users don't have to do the same work you had to do here