send_op fails when call AsyncGetVariable to get parameter server optimized weights

[typhoonzero]

The following code snippet will use an uninitialized variable. AsyncGetVariable only need the variable name to fetch the variable from parameter server, then initialized it by the return value of theAsyncGetVariable call.

auto* var = scope.FindVar(var_name);
SerializeToMessage(var_name, var, ctx, &req);