added compiler switch to select kernel's autograd method

Fix a copy and paste bug in gradient method service registry lookup (backward looked for forward impl...)
Signed-off-by: Thien Nguyen <nguyentm@ornl.gov>

examples/simple/gradients_optimization.cpp

+// Compile with:
+// qcor gradients_optimization.cpp
+// By default, kernel::autograd will use `central` gradient strategy.
+// This can be changed by using the `-autograd` qcor compiler flag:
+// e.g.,
+// qcor -autograd forward gradients_optimization.cpp
+// Options: forward, backward, central
+// Note: users may need to adjust the optimizer step size to guarantee
+// convergence.
 __qpu__ void ansatz(qreg q, double theta) {
   X(q[0]);
   Ry(q[1], theta);
@@ -33,7 +42,7 @@ int main(int argc, char **argv) {
   // Simple case 1: variational ansatz takes a single double
   {
     // Create the Optimizer (gradient-based)
-    auto optimizer = createOptimizer("nlopt", {{"nlopt-optimizer", "l-bfgs"}});
+    auto optimizer = createOptimizer("mlpack", {{"step-size", 1e-2}});
     ObjectiveFunction opt_function(
         [&](const std::vector<double> &x, std::vector<double> &dx) {
           auto q = qalloc(2);
@@ -54,7 +63,7 @@ int main(int argc, char **argv) {
   // Simple case 2: variational ansatz takes a vector<double>
   {
     // Create the Optimizer (gradient-based)
-    auto optimizer = createOptimizer("nlopt", {{"nlopt-optimizer", "l-bfgs"}});
+    auto optimizer = createOptimizer("mlpack", {{"step-size", 1e-2}});
     ObjectiveFunction opt_function(
         [&](const std::vector<double> &x, std::vector<double> &dx) {
           auto q = qalloc(2);
@@ -73,7 +82,7 @@ int main(int argc, char **argv) {
   }
   {
     // Create the Optimizer (gradient-based)
-    auto optimizer = createOptimizer("nlopt", {{"nlopt-optimizer", "l-bfgs"}});
+    auto optimizer = createOptimizer("mlpack", {{"step-size", 1e-2}});
     ObjectiveFunction opt_function(
         [&](const std::vector<double> &x, std::vector<double> &dx) {
           // Using kernel auto-gradient helper

handlers/token_collector/token_collector_util.cpp

@@ -296,7 +296,7 @@ std::string run_token_collector(
           mutable_parameters.begin(),
           "__" + kernel_name + "_" + counter_str + "__compute_functor");
 
-      auto src_code = __internal__::qcor::construct_kernel_subtype(
+      auto src_code = ::__internal__::qcor::construct_kernel_subtype(
           tmpss.str(), internal_kernel_function_name, mutable_arg_types,
           mutable_parameters, bufferNames);
 

runtime/objectives/gradient_function.cpp

@@ -15,8 +15,6 @@
 #include "xacc_internal_compiler.hpp"
 namespace qcor {
 namespace __internal__ {
-std::string DEFAULT_GRADIENT_METHOD = "central";
-
 std::shared_ptr<GradientFunction>
 get_gradient_method(const std::string &type,
                     std::shared_ptr<ObjectiveFunction> obj_func,

runtime/objectives/gradient_function.hpp

@@ -39,7 +39,6 @@ public:
 };
 
 namespace __internal__ {
-extern std::string DEFAULT_GRADIENT_METHOD;
 std::shared_ptr<GradientFunction>
 get_gradient_method(const std::string &type,
                     std::shared_ptr<ObjectiveFunction> obj_func,

runtime/objectives/gradients/FiniteDifference/FiniteDifferenceGradients.cpp

@@ -87,7 +87,7 @@ class KernelForwardDifferenceGradient : public KernelGradientService {
     m_kernel_eval = _kernel_eval;
     gradient_func = [&](const std::vector<double> &x,
                         double cost_val) -> std::vector<double> {
-      return run_gradient_strategy(x, cost_val, "forward", m_step, obs,
+      return run_gradient_strategy(x, cost_val, "forward", -m_step, obs,
                                    m_kernel_eval);
     };
   }
@@ -101,7 +101,7 @@ class KernelForwardDifferenceGradient : public KernelGradientService {
     gradient_func = [&](const std::vector<double> &x,
                         double cost_val) -> std::vector<double> {
       auto obs = m_objFunc->get_observable();
-      return run_gradient_strategy(x, cost_val, "forward", m_step, obs,
+      return run_gradient_strategy(x, cost_val, "forward", -m_step, obs,
                                    m_objFunc->get_kernel_evaluator());
     };
   }
@@ -129,7 +129,7 @@ class KernelBackwardDifferenceGradient : public KernelGradientService {
     m_kernel_eval = kernel_eval;
     gradient_func = [&](const std::vector<double> &x,
                         double cost_val) -> std::vector<double> {
-      return run_gradient_strategy(x, cost_val, "forward", m_step, obs,
+      return run_gradient_strategy(x, cost_val, "backward", m_step, obs,
                                    m_kernel_eval);
     };
   }

runtime/qcor.hpp

@@ -64,6 +64,9 @@ public:
 #endif
 #ifdef __internal__qcor__validate__execution
     xacc::internal_compiler::__validate_nisq_execution = true;
+#endif
+#ifdef __internal__qcor__autograd__method
+    xacc::internal_compiler::set_autograd_method(__internal__qcor__autograd__method);
 #endif
   }
 };

runtime/qrt/qrt.cpp

@@ -23,6 +23,12 @@
 #include "xacc_service.hpp"
 #include "qcor_observable.hpp"
 
+namespace qcor {
+namespace __internal__ {
+std::string DEFAULT_GRADIENT_METHOD = "central";
+}
+} // namespace qcor
+
 namespace xacc {
 namespace internal_compiler {
 // Extern vars:
@@ -101,6 +107,10 @@ std::string get_native_code(std::shared_ptr<qcor::CompositeInstruction> program,
   return get_qpu()->getNativeCode(program->as_xacc(), options);
 }
 
+void set_autograd_method(const std::string &method_name) {
+  qcor::__internal__::DEFAULT_GRADIENT_METHOD = method_name;
+}
+
 std::pair<bool, xacc::HeterogeneousMap>
 validate_backend_execution(std::shared_ptr<qcor::CompositeInstruction> program,
                            xacc::HeterogeneousMap options) {

runtime/qrt/qrt.hpp

@@ -268,6 +268,9 @@ void execute_pass_manager(
     std::shared_ptr<qcor::CompositeInstruction> optional_composite = nullptr);
 std::string get_native_code(std::shared_ptr<qcor::CompositeInstruction> program,
                             xacc::HeterogeneousMap options);
+
+void set_autograd_method(const std::string &method_name);
+
 // Hook to validate backend execution of a circuit
 // e.g., via circuit mirror technique
 // returns a pass/fail bool along with a untyped result data.
@@ -278,6 +281,10 @@ std::pair<bool, xacc::HeterogeneousMap> validate_backend_execution(
 } // namespace internal_compiler
 } // namespace xacc
 namespace qcor {
+namespace __internal__ {
+// Default gradient method to use with kernel's autograd method
+extern std::string DEFAULT_GRADIENT_METHOD;
+} // namespace __internal__
 // Ancilla qubit allocator:
 class AncQubitAllocator : public AllocEventListener, public QubitAllocator {
 public:

tools/driver/qcor.in

@@ -379,6 +379,8 @@ def main(argv=None):
         # Currently, it means enabling Pauli-randomized mirror circuit benchmarking:
         # i.e., executing a set of benchmarking circuits related to the original circuit that we know the expected bitstring
         parser.add_argument('-validate', metavar='', help='Enables backend execution validation via the mirror circuit technique.')
+        
+        parser.add_argument('-autograd', metavar='', help='Specifies the kernel autograd method. Options: central (default), forward, backward.')
 
         args = parser.parse_args(sys.argv)
 
@@ -646,7 +648,13 @@ def main(argv=None):
         if '-D_QCOR_FTQC_RUNTIME' in sys.argv:
             print('Execution validation cannot be used with FTQC runtime.')
             exit(1)
-
+    if '-autograd' in sys.argv[1:]:
+        sidx = sys.argv.index('-autograd')
+        autogradMethodName = sys.argv[sidx+1]
+        sys.argv.remove(autogradMethodName)
+        sys.argv.remove('-autograd')
+        sys.argv += ['-D__internal__qcor__autograd__method=\"'+autogradMethodName+'\"']   
+        
     # if not empty
     if '@QCOR_APPEND_PLUGIN_PATH@':
         sys.argv += ['-D__internal__qcor__compile__plugin__path=\"@QCOR_APPEND_PLUGIN_PATH@\"']