Wrote replacements for set_tensor() and scale_tensor() since the previous

versions were calling into cuDNN, however, the cuDNN functions for doing this
are horrifically slow, well over 100x slower than they should be, which is
surprising since these functions are so trivial.

dlib/dnn/cuda_dlib.cu

@@ -864,6 +864,38 @@ namespace dlib
             launch_kernel(_add_bias_gradient,max_jobs(grad.size()),grad.device(), gradient_input.device(), grad.size(), gradient_input.size());
         }
 
+    // ----------------------------------------------------------------------------------------
+
+        __global__ void _set_tensor(float* out, size_t n, const float val)
+        {
+            for (auto i : grid_stride_range(0, n))
+                out[i] = val;
+        }
+
+        void set_tensor (
+            tensor& t,
+            float value
+        )
+        {
+            launch_kernel(_set_tensor, max_jobs(t.size()), t.device(), t.size(), value);
+        }
+
+    // ----------------------------------------------------------------------------------------
+
+        __global__ void _scale_tensor(float* out, size_t n, const float val)
+        {
+            for (auto i : grid_stride_range(0, n))
+                out[i] *= val;
+        }
+
+        void scale_tensor (
+            tensor& t,
+            float value
+        )
+        {
+            launch_kernel(_scale_tensor, max_jobs(t.size()), t.device(), t.size(), value);
+        }
+
     // -----------------------------------------------------------------------------------
     // -----------------------------------------------------------------------------------
 

dlib/dnn/cuda_dlib.h

@@ -141,6 +141,18 @@ namespace dlib
             const tensor& v2
         );
 
+    // ------------------------------------------------------------------------------------
+
+        void set_tensor (
+            tensor& t,
+            float value
+        );
+
+        void scale_tensor (
+            tensor& t,
+            float value
+        );
+
     // ------------------------------------------------------------------------------------
 
         void multiply (

dlib/dnn/cudnn_dlibapi.cpp

@@ -289,32 +289,6 @@ namespace dlib
                                     dest.device()));
         }
 
-        void set_tensor (
-            tensor& t,
-            float value
-        )
-        {
-            if (t.size() == 0)
-                return;
-            CHECK_CUDNN(cudnnSetTensor(context(),
-                                 descriptor(t),
-                                 t.device_write_only(),
-                                 &value));
-        }
-
-        void scale_tensor (
-            tensor& t,
-            float value
-        )
-        {
-            if (t.size() == 0)
-                return;
-            CHECK_CUDNN(cudnnScaleTensor(context(),
-                                   descriptor(t),
-                                   t.device(),
-                                   &value));
-        }
-
         void assign_conv_bias_gradient (
             tensor& grad,
             const tensor& gradient_input

dlib/dnn/cudnn_dlibapi.h

@@ -89,26 +89,6 @@ namespace dlib
                   add into the dest tensor.
         !*/
 
-        void set_tensor (
-            tensor& t,
-            float value
-        );
-        /*!
-            ensures
-                - sets all elements in t equal to value.
-        !*/
-
-        void scale_tensor (
-            tensor& t,
-            float value
-        );
-        /*!
-            ensures
-                - scales all elements of t by the given value.  I.e. for all elements E in
-                  t, this function performs:
-                    - E = E*value
-        !*/
-
     // ------------------------------------------------------------------------------------
 
         void assign_conv_bias_gradient (

dlib/dnn/tensor.h

@@ -14,6 +14,22 @@
 
 namespace dlib
 {
+
+// ----------------------------------------------------------------------------------------
+
+    namespace cuda
+    {
+        void set_tensor (
+            tensor& t,
+            float value
+        );
+
+        void scale_tensor (
+            tensor& t,
+            float value
+        );
+    }
+
 // ----------------------------------------------------------------------------------------
 
     class tensor

dlib/test/dnn.cpp

@@ -339,6 +339,14 @@ namespace
         dlog << LINFO << mat(dest);
         matrix<float> truth1(3,4), truth2(3,4);
 
+        truth1 = 2;
+        DLIB_TEST(max(abs(truth1-mat(src))) < 1e-5);
+        src *= 2;
+        truth1 = 4;
+        DLIB_TEST(max(abs(truth1-mat(src))) < 1e-5);
+        src = 2;
+
+
         truth1 = 7;
         truth2 = 7, 10,  7,  7,
         7, 10,  7,  7,