Compute mean and variance

include/layers/batch_norm.cuh

@@ -74,8 +74,12 @@ class BatchNorm : public WeightedLayer {
     float* d_output;
 
     float* d_mean;
+    float* d_mean_sub;
     float* d_sqrt_var;
 
+    float* d_length;
+    float* d_epsilon;
+
     float* d_weights;
     float* d_biases;