Allow kernel_generator to be stateful

In python kernel generator is a user-defined callable.
We need to capture Python object of that callable in
kernel generator provided for each benchmark.

To this end, nvbench::benchmark has been modified to have member of
kernel_generator type (must be copy-constructable). Constructor acquires
an optional parameter of type `kernel_generator` with default value
of default-contstructed instance.

nvbench::runner was modified to store kernel_generator instance as well.
Its run method creates a fresh copy of stored instance for each invocation,
just as it was happening before.

nvbench tests/examples pass with this change.

Author: oleksandr-pavlyk

nvbench/benchmark.cuh

@@ -58,12 +58,16 @@ struct benchmark final : public benchmark_base
 
   static constexpr std::size_t num_type_configs = nvbench::tl::size<type_configs>{};
 
-  benchmark()
+  benchmark(kernel_generator kgen = {})
       : benchmark_base(type_axes{})
+      , m_kernel_generator(kgen)
   {}
 
 private:
-  std::unique_ptr<benchmark_base> do_clone() const final { return std::make_unique<benchmark>(); }
+  std::unique_ptr<benchmark_base> do_clone() const final
+  {
+    return std::make_unique<benchmark>(this->m_kernel_generator);
+  }
 
   void do_set_type_axes_names(std::vector<std::string> names) final
   {
@@ -72,10 +76,12 @@ private:
 
   void do_run() final
   {
-    nvbench::runner<benchmark> runner{*this};
+    nvbench::runner<benchmark> runner{*this, this->m_kernel_generator};
     runner.generate_states();
     runner.run();
   }
+
+  kernel_generator m_kernel_generator;
 };
 
 } // namespace nvbench

nvbench/runner.cuh

@@ -54,8 +54,9 @@ struct runner : public runner_base
   using type_configs                            = typename benchmark_type::type_configs;
   static constexpr std::size_t num_type_configs = benchmark_type::num_type_configs;
 
-  explicit runner(benchmark_type &bench)
+  explicit runner(benchmark_type &bench, kernel_generator kgen = {})
       : runner_base{bench}
+      , m_kernel_generator{kgen}
   {}
 
   void run()
@@ -98,7 +99,8 @@ private:
             self.run_state_prologue(cur_state);
             try
             {
-              kernel_generator{}(cur_state, type_config{});
+              auto kernel_generator_copy = self.m_kernel_generator;
+              kernel_generator_copy(cur_state, type_config{});
               if (cur_state.is_skipped())
               {
                 self.print_skip_notification(cur_state);
@@ -115,6 +117,8 @@ private:
         ++type_config_index;
       });
   }
+
+  kernel_generator m_kernel_generator;
 };
 
 } // namespace nvbench