add the xacc accelerator demo

#include "qcor.hpp"

#include "xacc.hpp"

// Demo steps (show off backend extensibility via XACC)

//

// qcor xacc_demo.cpp

// ./a.out 

// ./a.out --execute

// ./a.out --accelerator aer

// ./a.out --accelerator ibm:ibmq_jakarta

// ./a.out --accelerator aer:ibmq_jakarta --execute

// ./a.out --accelerator honeywell:HQS-LT-S1-APIVAL

using namespace qcor::arg;

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

  // Command line argument setup

  std::string default_qpu = "qpp";

  add_argument("--accelerator").default_value(default_qpu);

  add_argument("--execute").default_value(false).implicit_value(true);

  add_argument("--shots").default_value(100).action(

      [](const std::string& value) { return std::stoi(value); });

  parse_args(argc, argv);

  // Get user input

  auto qpu = get_argument("--accelerator");

  auto shots = get_argument<int>("--shots");

  auto exec = get_argument<bool>("--execute");

  print("You chose the", qpu, "accelerator.");

  // This is the key piece. Ultimately under the hood 

  // for OpenQASM3, we delegate to a call like this 

  auto backend = xacc::getAccelerator(qpu, {{"shots", shots}});

  // EXAMPLES, Bell State and GHZ State

  // Execute or Print Native Code

  {  // BELL STATE

    const int n_qubits = 2;

    xacc::IRBuilder builder;

    builder.h(0);

    builder.cnot(0, 1);

    for (int i = 0; i < n_qubits; i++) builder.mz(i);

    auto composite = builder.to_ir();

    // Exec if told to, else print the native code

    if (exec) {

      auto accelerator_buffer =

          std::make_shared<xacc::AcceleratorBuffer>("q", n_qubits);

      backend->execute(accelerator_buffer, composite);

      accelerator_buffer->print();

    } else {

      print("Native Code:\n", backend->getNativeCode(composite, {{"format", "qasm"}}));

    }

  }

  {  // GHZ STATE

    const int n_qubits = 3;

    xacc::IRBuilder builder;

    builder.h(0);

    for (int i = 0; i < n_qubits - 1; i++) builder.cnot(i, i + 1);

    for (int i = 0; i < n_qubits; i++) builder.mz(i);

    auto composite = builder.to_ir();

    // Exec if told to, else print the native code

    if (exec) {

      auto accelerator_buffer =

          std::make_shared<xacc::AcceleratorBuffer>("q", n_qubits);

      backend->execute(accelerator_buffer, composite);

      accelerator_buffer->print();

    } else {

      print("Native Code:\n", backend->getNativeCode(composite, {{"format", "qasm"}}));

    }

  }

  return 0;

}