[Verifier] Add a function to print out the difference of two circuits

[xumingkuan]

This PR adds these functions:

  /**
   * A helper method to extract the difference between two circuits, used
   * by the equivalent() function above. The two circuits are expected
   * to differ by only one small circuit transformation.
   * @param ctx The context for both circuits.
   * @param circuit1 The first input circuit.
   * @param circuit2 The second input circuit.
   * @param output_circuit1 Return the different part for circuit1.
   * Should pass in a variable initialized to nullptr to store the return value.
   * @param output_circuit2 Return the different part for circuit2.
   * Should pass in a variable initialized to nullptr to store the return value.
   * @param verbose Print logs to the screen.
   * @return True iff the function succeeded.
   */
  static bool extract_difference(Context *ctx, const CircuitSeq *circuit1,
                                 const CircuitSeq *circuit2,
                                 std::unique_ptr<CircuitSeq> &output_circuit1,
                                 std::unique_ptr<CircuitSeq> &output_circuit2,
                                 bool verbose = false);
  /**
   * Extract the difference between two circuits to a string.
   * @param ctx The context for both circuits.
   * @param circuit1 The first input circuit.
   * @param circuit2 The second input circuit.
   * @param columns_before_midline Print out a character '|' at this location
   * for each line to separate two circuits.
   * @param param_precision The parameter precision for the output.
   * @return A string side-by-side for the difference.
   */
  static std::string difference_str(Context *ctx, const CircuitSeq *circuit1,
                                    const CircuitSeq *circuit2,
                                    int columns_before_midline = 40,
                                    int param_precision = kDefaultQASMParamPrecision);
  static std::string difference_str(const Graph *circuit1,
                                    const Graph *circuit2,
                                    int columns_before_midline = 40,
                                    int param_precision = kDefaultQASMParamPrecision);

Usage example:

#include "quartz/verifier/verifier.h"
int main() {
  ParamInfo param_info;
  Context ctx({GateType::rz, GateType::input_qubit, GateType::input_param,
               GateType::mult, GateType::pi},
              1, &param_info);

  std::string str = "OPENQASM 2.0;\n"
                    "include \"qelib1.inc\";\n"
                    "qreg q[1];\n"
                    "rz(pi*1/2) q[0];\n"
                    "rz(pi*2/3) q[0];\n"
                    "rz(pi) q[0];\n"
                    "rz(pi/4) q[0];\n";
  auto graph1 = Graph::from_qasm_str(&ctx, str);
  str = "OPENQASM 2.0;\n"
        "include \"qelib1.inc\";\n"
        "qreg q[1];\n"
        "rz(pi/2) q[0];\n"
        "rz(2*pi/3) q[0];\n"
        "rz(pi/2) q[0];\n"
        "rz(pi/4) q[0];\n";
  auto graph2 = Graph::from_qasm_str(&ctx, str);
  std::cout << Verifier::difference_str(graph1.get(), graph2.get());
  return 0;
}

Output:

rz(pi*1) q[0];                          | rz(pi*1/2) q[0];