message: add comments for IBM backend specific methods
  (only used by TriQ for noise-aware placement for IBM backends)
diff: @@ -336,16 +336,23 @@ public:
  return result;
  }
  + // Query Fidelity information:
  + virtual size_t nQubits() const override { return m_qubitLabels.size(); }
  +
  + // These methods are used for TriQ placement.
  + // We don't support them in this noise model impl.
  virtual double gateErrorProb(xacc::quantum::Gate &gate) const override {
  return 0.0;
  }
  - // Query Fidelity information:
  - virtual size_t nQubits() const override { return 0; }
  +
  virtual std::vector<double> averageSingleQubitGateFidelity() const override {
  + xacc::error("Unsupported");
  return {};
  }
  +
  virtual std::vector<std::tuple<size_t, size_t, double>>
  averageTwoQubitGateFidelity() const override {
  + xacc::error("Unsupported");
  return {};
  }
  