Add device.Wait() in fetch_op (#5141)

paddle/operators/fetch_op.cc

@@ -52,6 +52,7 @@ class FetchOp : public framework::OperatorBase {
     // FIXME(yuyang18): Should we assume the fetch operator always generate
     // CPU outputs?
     dst_item.CopyFrom(src_item, platform::CPUPlace(), dev_ctx);
+    dev_ctx.Wait();
     dst_item.set_lod(src_item.lod());
 
     VLOG(3) << "Fetch variable " << fetch_var_name << " to " << out_name;