Restructure cuda backend

src/backends/cuda/kernels/activation_functions.cu

@@ -0,0 +1,30 @@
+#include "activation_functions.cuh"
+#include "cuda_helper.cuh"
+
+using namespace CUDANet;
+
+__global__ void Kernels::sigmoid(
+    const float* __restrict__ src,
+    float* __restrict__ dst,
+    const unsigned int len
+) {
+    int stride = gridDim.x * blockDim.x;
+    int tid    = blockDim.x * blockIdx.x + threadIdx.x;
+
+    for (int i = tid; i < len; i += stride) {
+        dst[i] = 1.0 / (1.0 + exp(-src[i]));
+    }
+}
+
+__global__ void Kernels::relu(
+    const float* __restrict__ src,
+    float* __restrict__ dst,
+    const unsigned int len
+) {
+    int stride = gridDim.x * blockDim.x;
+    int tid    = blockDim.x * blockIdx.x + threadIdx.x;
+
+    for (int i = tid; i < len; i += stride) {
+        dst[i] = src[i] < 0.0 ? 0.0 : src[i];
+    }
+}