Migrate concat layer

src/backends/cuda/layers/concat.cu

@@ -0,0 +1,31 @@
+#include "concat.hpp"
+#include "cuda_helper.cuh"
+
+using namespace CUDANet::Layers;
+
+void Concat::initCUDA() {
+    d_output = nullptr;
+    CUDA_CHECK(
+        cudaMalloc((void**)&d_output, sizeof(float) * (inputASize + inputBSize))
+    );
+}
+
+void Concat::delCUDA() {
+    cudaFree(d_output);
+}
+
+float* Concat::forwardCUDA(const float* d_input_A, const float* d_input_B) {
+    CUDA_CHECK(cudaMemcpy(
+        d_output, d_input_A, sizeof(float) * inputASize,
+        cudaMemcpyDeviceToDevice
+    ));
+
+    CUDA_CHECK(cudaMemcpy(
+        d_output + inputASize, d_input_B, sizeof(float) * inputBSize,
+        cudaMemcpyDeviceToDevice
+    ));
+
+    CUDA_CHECK(cudaDeviceSynchronize());
+
+    return d_output;
+}