Embed ATen/core/CMakeLists.txt into its parent (#38426)
Summary:
This file were separated from main CMakeLists.txt to enable mobile builds, but at the moment it is only referenced from CMakeLists.txt in parents folder
This is a preparatory step to move `jit_core_sources`,`jit_core_headers` to build_variables.bzl
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38426
Test Plan: CI
Differential Revision: D21567389
Pulled By: malfet
fbshipit-source-id: e6340fad1da75aa3e24d6c340df0c3e1e1957595
