Incorrect std::vector differentiation in the reverse mode

[gojakuch]

#include "clad/Differentiator/Differentiator.h"
#include "clad/Differentiator/STLBuiltins.h"
#include <vector>
#include <iostream>

double fn(double x, double y) {
  std::vector<double> a;
  a.push_back(x);
  a.push_back(x);
  a[1] = x*x;
  return a[1];
}

int main(int argc, char* argv[]) {
    double dx=0, dy=0;
    auto df = clad::gradient(fn, "x, y");
    std::cout << fn(3, 4) << '\n';
    df.execute(3, 4, &dx, &dy);
    std::cout << dx << ' ' << dy << '\n';
}

expected output:
9
6 0
output:
9
1 0

my assumption as to why this happens is that we don't match the constructor forw and pullback properly. but I'm not sure, it could be that other things are not matched properly as well or that it's another kind of problem.

[gojakuch]

this might be related to #1067