Easy way to grab just lower triangular values as vector from semi-metric distance calculations?

[brendanwallison]

Hello, after running a Pairwise distance calculation with a semi-metric distance metric this package returns an NxN matrix even though it is symmetric across the diagonal.

For my purposes I need to extract just the values from the lower triangular portion for further operation. This sounds trivial (and I'm sure that it is), but it is in a differentiable loss function where I'm using Zygote, which raises an error when mutating arrays*. So if I use tril or a list comprehension to extract the values, the code errors (eg https://stackoverflow.com/questions/50651781/extract-lower-triangle-portion-of-a-matrix). The vec command doesn't, but it doesn't give me what I need.

I know that in the background if the distance is semi-metric then it's not actually calculating the redundant portion. I'm wondering if there is a quick and easy way to grab the actually unique values of the distance metric from one of these semi-metric structures, since it sounds like that may be the form it's actually in before constructing the matrix.

*In a related note, Zygote errors for colwise distance calculations but not pairwise for the same reason. This is less urgent, but is there a way to perform the colwise calculation without mutations that break Zygote automatic differentiation?