Restructure cuda backend

src/backends/cuda/layers/add.cu

@@ -0,0 +1,28 @@
+#include "add.hpp"
+#include "matmul.cuh"
+#include "cuda_helper.cuh"
+
+using namespace CUDANet::Layers;
+
+void Add::initCUDA() {
+    d_output = nullptr;
+    CUDA_CHECK(cudaMalloc((void**)&d_output, sizeof(float) * inputSize));
+
+    gridSize = (inputSize + BLOCK_SIZE - 1) / BLOCK_SIZE;
+}
+
+void Add::delCUDA() {
+    cudaFree(d_output);
+}
+
+float* Add::forwardCUDA(const float* d_inputA, const float* d_inputB) {
+
+    Kernels::vec_vec_add<<<gridSize, BLOCK_SIZE>>>(
+        d_inputA, d_inputB, d_output, inputSize
+    );
+    CUDA_CHECK(cudaGetLastError());
+    CUDA_CHECK(cudaDeviceSynchronize());
+
+    return d_output;
+
+}