Ensure test data are initialized

We have a few instances, mostly in the tests, where we do something like this:

void init_input_matrix(T *a) {
    unsigned int seed = 0;
    if constexpr (std::is_same_v<T, __half>) {
      < init for __half>
    } else if constexpr (std::is_same_v<T, unsigned int>) {
      < init for 1bit>
    }

We should ensure there is a matching if statement for all input types, e.g. by adding a final else that throws a runtime error. We do this in some places already, but not everywhere.