Created by: wangkuiyi

Currently in paddle/platform/place.h we have

void set_place(const Place &);
const Place &get_place();

It looks convenient and flexible being able to create a Tensor on the default device. However, it also makes it possible that some tensors created on an explicitly specified device cannot work together with those on the default device if the specified one differs from the default one.

This can be summarized as the evil of mutable global variables. Let's remove them, and require that places are always specified explicitly.