Made cuBLAS and cuDNN automatically switch their library handles to the

currently active device id if the user changes the active device via a call to
cudaSetDevice().

dlib/dnn/cublas_dlibapi.cpp

@@ -6,6 +6,7 @@
 #ifdef DLIB_USE_CUDA
 
 #include "cublas_dlibapi.h"
+#include "cuda_utils.h"
 
 #include <cublas_v2.h>
 
@@ -52,6 +53,7 @@ namespace dlib
             cublas_context()
             {
                 CHECK_CUBLAS(cublasCreate(&handle));
+                CHECK_CUDA(cudaGetDevice(&device_id));
             }
             ~cublas_context()
             {
@@ -59,18 +61,27 @@ namespace dlib
             }
 
             cublasHandle_t get_handle (
-            ) const { return handle; }
+            )  
+            { 
+                // Check if the active device for the current thread changed.  If so then
+                // regenerate our cuBLAS handle so it will use the currently selected
+                // device.
+                int new_device_id;
+                CHECK_CUDA(cudaGetDevice(&new_device_id));
+                if (new_device_id != device_id)
+                {
+                    CHECK_CUBLAS(cublasDestroy(handle));
+                    CHECK_CUBLAS(cublasCreate(&handle));
+                }
+                return handle; 
+            }
 
         private:
 
             cublasHandle_t handle;
+            int device_id;
         };
 
-        // TODO, there should probably be some function that is like dlibCudaSetDevice().
-        // Because people will call cudaSetDevice() expecting to set the device but for
-        // cuBLAS and cuDNN, since they have these handles, they will keep using the old
-        // devices.  So we should have something that resets these handles and does a
-        // "dlibCudaSetDevice()"
         static cublasHandle_t context()
         {
             thread_local cublas_context c;

dlib/dnn/cudnn_dlibapi.cpp

@@ -70,6 +70,7 @@ namespace dlib
             cudnn_context()
             {
                 CHECK_CUDNN(cudnnCreate(&handle));
+                CHECK_CUDA(cudaGetDevice(&device_id));
             }
 
             ~cudnn_context()
@@ -78,10 +79,24 @@ namespace dlib
             }
 
             cudnnHandle_t get_handle (
-            ) const { return handle; }
+            ) 
+            { 
+                // Check if the active device for the current thread changed.  If so then
+                // regenerate our cuDNN handle so it will use the currently selected
+                // device.
+                int new_device_id;
+                CHECK_CUDA(cudaGetDevice(&new_device_id));
+                if (new_device_id != device_id)
+                {
+                    CHECK_CUDNN(cudnnDestroy(handle));
+                    CHECK_CUDNN(cudnnCreate(&handle));
+                }
+                return handle; 
+            }
 
         private:
             cudnnHandle_t handle;
+            int device_id;
         };
 
         static cudnnHandle_t context()