Summary: When merging centroids together, we recalculate the mean and weight. This can sometimes lead to inaccuracy due to floating point arithmetic. In a bad scenario, this can actually lead to the digest being unsorted, which triggers assertions in the code. To fix this, sort the digest at the end of merge. Since this is bounded to 100 elements that are mostly sorted, this appears to be cheap according to benchmarks.

Reviewed By: yfeldblum

Differential Revision: D9213936

fbshipit-source-id: 5a2978a1b759b70206aef57245bef193aed65efd