Implement avg pooling

include/layers/avg_pooling.cuh

@@ -0,0 +1,38 @@
+#ifndef CUDANET_AVG_POOLING_H
+#define CUDANET_AVG_POOLING_H
+
+#include "activation.cuh"
+#include "layer.cuh"
+
+namespace CUDANet::Layers {
+
+class AvgPooling2D : public SequentialLayer {
+  public:
+    AvgPooling2D(
+        int            inputSize,
+        int            nChannels,
+        int            poolingSize,
+        int            stride,
+        ActivationType activationType
+    );
+    ~AvgPooling2D();
+
+    float* forward(const float* d_input);
+
+  private:
+    int inputSize;
+    int nChannels;
+    int poolingSize;
+    int stride;
+
+    int outputSize;
+    int gridSize;
+
+    float* d_output;
+
+    Activation activation;
+};
+
+}  // namespace CUDANet::Layers
+
+#endif  // CUDANET_AVG_POOLING_H