Resolve regression from introduction of AER::Config (Qiskit#1747)

This commit reduces redundant copy in `AER::Transpile::CircuitOptimization`.
Since 0.12.0, configuration is `AER::Config` instead of `json_t`. This new class
has many fields and then its copy overheads become high. Reduction of
redundant copy improves performance especially for low-qubit simulation.

releasenotes/notes/avoid_copy_of_config-7f7891864c1a1bd0.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Performance regression due to introduction of ``AER::Config`` is fixed.
+    This class has many fields but is frequently copied in ``AER::Transpile::CircuitOptimization``.
+    Originally ``json_t`` (former class for configuration) was also frequently copied but
+    it does have entries in most cases and then this copy overhead is not a problem.
+    With this fix, ``AER::Transpile::CircuitOptimization`` does not copy ``AER::Config``.

src/transpile/circuitopt.hpp

@@ -40,14 +40,11 @@ class CircuitOptimization {
                                 const Operations::OpSet &opset,
                                 ExperimentResult &result) const = 0;
 
-  virtual void set_config(const Config &config);
+  virtual void set_config(const Config &config){};
 
 protected:
-  Config config_;
 };
 
-void CircuitOptimization::set_config(const Config &config) { config_ = config; }
-
 //-------------------------------------------------------------------------
 } // end namespace Transpile
 } // end namespace AER

src/transpile/fusion.hpp

@@ -791,7 +791,7 @@ void Fusion::set_config(const Config &config) {
     threshold = config.fusion_threshold.value();
 
   for (std::shared_ptr<Fuser> &fuser : fusers)
-    fuser->set_config(config_);
+    fuser->set_config(config);
 
   if (config.fusion_allow_kraus.has_value())
     allow_kraus = config.fusion_allow_kraus.value();