Merge pull request BVLC#3410 from ghost/patch-2

sigmoid fix: calculate by tanh instead of exp (cpp)

src/caffe/layers/sigmoid_layer.cu

@@ -8,7 +8,7 @@ namespace caffe {
 template <typename Dtype>
 __global__ void SigmoidForward(const int n, const Dtype* in, Dtype* out) {
   CUDA_KERNEL_LOOP(index, n) {
-    out[index] = 1. / (1. + exp(-in[index]));
+    out[index] = 0.5 * tanh(0.5 * in[index]) + 0.5;
   }
 }