I see now that the Kronecker Normal may be what I was thinking of:
https://docs.pymc.io/api/distributions/multivariate.html#pymc3.distributions.multivariate.KroneckerNormal
Some docs are provided here:
https://docs.pymc.io/notebooks/GP-Kron.html
I also found this article on the subject:
https://www.sciencedirect.com/science/article/pii/S0377042712003810