Move stack.h to ATen/core (#16247)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16247
Stack is going to be used by the c10 dispatcher.
This just moves the file, also changing the namespace turned out to be more complicated than I thought, I'll leave the namespace for now.
Reviewed By: ezyang
Differential Revision: D13774189
fbshipit-source-id: 66aeee36425e0ea2b3a4f8159604f38572306d57
