Changed the trainer threading code to use dlib::thread_pool instead of

std::async() since std::async creates new threads with each invocation, which
in turn causes objects with thread_local storage duration to be reconstructed
each time.  This is problematic because CUDA context objects for cublas and
cudnn get reconstructed over and over, slowing things down and generally using
more resources than should be used.

dlib/dnn/trainer.h

@@ -526,8 +526,7 @@ namespace dlib
             label_type pick_which_run_update;
             job_t next_job;
 
-            std::vector<std::future<double>> losses(devices.size());
-            std::vector<std::future<void>> update_futs(devices.size());
+            std::vector<dlib::future<double>> losses(devices.size());
 
             std::vector<tt::multi_device_tensor_averager> averagers;
             // An array of all the parameter tensors in the first network.  We will
@@ -536,6 +535,8 @@ namespace dlib
             std::vector<tensor*> reference_params;
             visit_layer_parameters(devices[0]->net, [&](size_t, tensor& t) { reference_params.push_back(&t); });
 
+            thread_pool tp(devices.size());
+
 
             size_t iteration = 0;
             while(job_pipe.dequeue(next_job))
@@ -545,7 +546,7 @@ namespace dlib
                 // right version for unsupervised or supervised training based on the type
                 // of label_type.
                 for (size_t i = 0; i < devices.size(); ++i)
-                    losses[i] = std::async(std::launch::async,[&,i](){ return compute_parameter_gradients(i, next_job, pick_which_run_update); });
+                    tp.add_task_by_value([&,i](double& loss){ loss = compute_parameter_gradients(i, next_job, pick_which_run_update); }, losses[i]);
                 // aggregate loss values from all the network computations.
                 double theloss = 0;
                 for (auto&& loss : losses)
@@ -596,10 +597,9 @@ namespace dlib
 
                 // Now apply all the updates to each device.
                 for (size_t i = 0; i < devices.size(); ++i)
-                    update_futs[i] = std::async(std::launch::async, [&,i](){ if (next_job.have_data[i]) update_parameters(i); });
+                    tp.add_task_by_value([&,i](){ if (next_job.have_data[i]) update_parameters(i); });
                 // and wait for the updates to all happen.
-                for (auto&& f : update_futs)
-                    f.wait();
+                tp.wait_for_all_tasks();
 
 
                 // Evey now and then force all the parameters to be the same just to make