Added a simple QAOA placement example

/// qaoa_placement.cpp: Topology placement example

/// qcor -qpu ibm:ibmq_paris  qaoa_placement.cpp -print-final-submission

__qpu__ void qaoa_maxcut(qreg q, std::vector<double> gamma,

                  std::vector<double> beta,

                  std::vector<std::pair<int, int>> graph_edges) {

  auto nQubits = q.size();

  int p = gamma.size();

  // Start of in the uniform superposition

  H(q);

  // Loop over qaoa steps

  for (int step = 0; step < p; step++) {

    // Loop over graph edges

    for (int i = 0; i < graph_edges.size(); i++) {

      auto [node1, node2] = graph_edges[i];

      auto ham_operator =  Z(node1) * Z(node2);

      // trotterize

      exp_i_theta(q, gamma[step], ham_operator);

    }

    // Add the reference hamiltonian term (mixer)

    for (int i = 0; i < nQubits; i++) {

      auto ref_ham_term = X(i);

      exp_i_theta(q, beta[step], ref_ham_term);

    }

  }

}

int main(int argc, char **argv) {

  auto q = qalloc(5);

  // Ring graph

  std::vector<std::pair<int, int>> graph{{0, 1}, {1, 2}, {2, 3}, {3, 4}};

  // One step

  int p = 1;

  std::vector<double> gammas{1.0};

  std::vector<double> betas{1.0};

  qaoa_maxcut::print_kernel(q, gammas, betas, graph);

  qaoa_maxcut(q, gammas, betas, graph);

}