fix gpu_img_conv2 float precision issue

paddle/gserver/tests/test_NetworkCompare.cpp

@@ -261,9 +261,12 @@ TEST(Compare, img_conv2) {
   std::string config_file_a = "./gserver/tests/img_conv_a.conf";
   std::string config_file_b = "./gserver/tests/img_conv_c.conf";
   bool useGpu = FLAGS_use_gpu;
+  double eps = FLAGS_checkgrad_eps;
   FLAGS_use_gpu = true;
+  FLAGS_checkgrad_eps = 1e-3;
   compareNetwork(config_file_a, config_file_b);
   FLAGS_use_gpu = useGpu;
+  FLAGS_checkgrad_eps = eps;
 }
 #endif