Need an interface to get Place instance

[Yancey1989]

When I call memory::Copy in Op Kernel, the code like:

paddle::memory::Copy<Place, Place>(
            boost::get<Place>(ctx.GetPlace()), static_cast<void*>(dst),
            boost::get<Place>(ctx.GetPlace()), static_cast<const void*>(src),
            num);

How about add an interface in framework::ExecutionContext named Place():

template<typename T>
inline const T& Place() {
    return boost::get<T>(device_context_->GetPlace());
}

So the code will be chaned into:

paddle::memory::Copy<Place, Place>(
            ctx.Place<Place>(), static_cast<void*>(dst),
            ctx.Place<Place>(), static_cast<const void*>(src),
            num);