Use VisitorBase::getZeroInit() or ConstantFolder::synthesizeLiteral() for assignments and initializations of variables

[kchristin22]

Currently not all variables initialized or assigned to a value use VisitorBase::getZeroInit() or ConstantFolder::synthesizeLiteral() accordingly. Hence, the code is mixed or these functions are not used at all, as the Forward Mode tests indicate.

Example:

#include "clad/Differentiator/Differentiator.h"

double fn(const double* arr, size_t n) {
  double sum = 0;
  for (size_t i=0; i < n; ++i) {
    sum += arr[i];
  }
  return sum;
}


int main() {
  auto grad = clad::gradient(fn);
  return 0;
}

The code produced is:

void fn_grad(const double *arr, size_t n, double *_d_arr, size_t *_d_n) {
    size_t _d_i = 0UL;
    size_t i = 0UL;
    clad::tape<double> _t1 = {};
    double _d_sum = 0.;
    double sum = 0; // should be 0.
    unsigned long _t0 = 0UL;
    for (i = 0; ; ++i) {
        {
            if (!(i < n))
                break;
        }
        _t0++;
        clad::push(_t1, sum);
        sum += arr[i];
    }
    _d_sum += 1;
    for (;; _t0--) {
        {
            if (!_t0)
                break;
        }
        --i;
        {
            sum = clad::pop(_t1);
            double _r_d0 = _d_sum;
            _d_arr[i] += _r_d0;
        }
    }
}

As the comment indicates, the var sum should be initialized as so:

    double sum = 0.;