use FLAGS_use_mkldnn to prevent unnecessary attrs copy (#28146)

paddle/fluid/imperative/prepared_operator.cc

@@ -20,6 +20,8 @@
 #include "paddle/fluid/imperative/infer_shape_context.h"
 #include "paddle/fluid/imperative/infer_var_type_context.h"
 
+DECLARE_bool(use_mkldnn);
+
 namespace paddle {
 namespace imperative {
 
@@ -91,8 +93,10 @@ PreparedOp PrepareOpImpl(const NameVarMap<VarType>& ins,
   // MKLDNN variant of code reads attributes in some of GetKernelTypeForVar and
   // GetKernelType functions, so we need to copy the attributes there.
   // Const qualifier of Attrs had to be discarded to overwrite it.
+  if (FLAGS_use_mkldnn) {
     auto& mutable_op_attrs = const_cast<framework::AttributeMap&>(op.Attrs());
     mutable_op_attrs = attrs;
+  }
 #endif
   auto expected_kernel_key =
       op.GetExpectedKernelType(DygraphExecutionContext<VarType>(