Fix includes for ATen/core/stack.h (#16462)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16462
This file was moved, now we change the includes to the new location and remove the proxy header.
Reviewed By: ezyang
Differential Revision: D13847279
fbshipit-source-id: 4617d52fdcfe785cb7b2154460a6686c437abd8b
