Merge branch 'optest-bug' into 'master'

Fix op_test_util bug when there is no operation.

See merge request !1021

mace/ops/buffer_to_image_benchmark.cc

@@ -50,8 +50,6 @@ void FilterBufferToImage(int iters,
                    b2i_output);
   };
 
-  // Warm-up
-  net.Setup(D);
   for (int i = 0; i < 5; ++i) {
     transform_func();
   }

mace/ops/ops_test_util.cc

@@ -188,6 +188,7 @@ bool OpsTestNet::Setup(mace::DeviceType device) {
       }
     }
   }
+  if (!op_defs_.empty()) {
     auto op_def = op_defs_.back();
     for (int i = 0; i < op_def.output_size(); ++i) {
       ws_.RemoveTensor(op_def.output(i));
@@ -199,6 +200,7 @@ bool OpsTestNet::Setup(mace::DeviceType device) {
         output_info->set_data_type(DataType::DT_FLOAT);
       }
     }
+  }
   MemoryOptimizer mem_optimizer;
   net_ = make_unique<SerialNet>(
       op_registry_.get(),