adam op should not create tmp var in compute

paddle/fluid/operators/optimizers/adam_op.h

@@ -423,6 +423,7 @@ class AdamOpKernel : public framework::OpKernel<T> {
         }
       }
 
+      framework::SelectedRows cpu_grad_merge;
       const framework::SelectedRows* grad_merge_ptr;
       if (is_strict_sorted) {
         grad_merge_ptr = &grad;
@@ -430,12 +431,16 @@ class AdamOpKernel : public framework::OpKernel<T> {
         // merge duplicated rows if any.
         // The rows of grad_merge have been sorted inside MergeAdd functor
         scatter::MergeAdd<DeviceContext, T> merge_func;
+        if (platform::is_cpu_place(ctx.GetPlace())) {
+          grad_merge_ptr = &cpu_grad_merge;
+        } else {
+          // FIXME(qiao): GPU also need to fix this
           auto* grad_merge_var = const_cast<framework::Scope&>(ctx.scope())
                                      .Var()
                                      ->GetMutable<framework::SelectedRows>();
+        }
         merge_func(ctx.template device_context<DeviceContext>(), grad,
-                   grad_merge_var, true);
-        grad_merge_ptr = grad_merge_var;
+                   grad_merge_ptr, true);
       }
 
       auto& grad_merge = *grad_merge_ptr;