adam op handle grad.rows().size == 0 condition

paddle/fluid/operators/adam_op.h

@@ -282,6 +282,10 @@ class AdamOpKernel : public framework::OpKernel<T> {
     } else if (grad_var->IsType<framework::SelectedRows>()) {
       auto& grad =
           Ref(ctx.Input<framework::SelectedRows>("Grad"), "Must set Grad");
+      if (grad.rows().size() == 0) {
+        VLOG(3) << "grad row size is 0!!";
+        return;
+      }
       // merge duplicated rows if any.
       scatter::MergeAdd<DeviceContext, T> merge_func;
       auto grad_merge =