Instantiate caffe_cpu_sign for float and double

src/caffe/util/math_functions.cpp

@@ -410,4 +410,10 @@ void caffe_gpu_asum<double>(const int n, const double* x, double* y) {
   CUBLAS_CHECK(cublasDasum(Caffe::cublas_handle(), n, x, 1, y));
 }
 
+template <>
+void caffe_cpu_sign<float>(const int n, const float* x, float* y);
+
+template <>
+void caffe_cpu_sign<double>(const int n, const double* x, double* y);
+
 }  // namespace caffe