Fix number of parallel shots for statevector and unitary sims (#725)

* Prevent statevector and unitary controller from running parallel shots

This was causing bugs where number of shots of a statevector and unitary simulators would be the number of CPU cores.

releasenotes/notes/fix-controller-parallel-shots-d32967c029259790.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed bug with statevector and unitary simulators running a number of (parallel)
+    shots equal to the number of CPU threads instead of only running a single shot.

src/controllers/statevector_controller.hpp

@@ -137,6 +137,10 @@ void StatevectorController::set_config(const json_t& config) {
   // Set base controller config
   Base::Controller::set_config(config);
 
+  // Override max parallel shots to be 1 since this should only be used
+  // for single shot simulations
+  Base::Controller::max_parallel_shots_ = 1;
+
   // Add custom initial state
   if (JSON::get_value(initial_state_, "initial_statevector", config)) {
     // Check initial state is normalized

src/controllers/unitary_controller.hpp

@@ -129,6 +129,10 @@ void UnitaryController::set_config(const json_t &config) {
   // Set base controller config
   Base::Controller::set_config(config);
 
+  // Override max parallel shots to be 1 since this should only be used
+  // for single shot simulations
+  Base::Controller::max_parallel_shots_ = 1;
+
   // Add custom initial unitary
   if (JSON::get_value(initial_unitary_, "initial_unitary", config)) {
     // Check initial state is unitary