Fix install location for ATen_CORE_HEADERS by avoiding relative paths (#27449)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/20046
While installing, `aten/src/ATen` is shortened to just `ATen` so these relative paths become `/usr/local/include/ATen/core/../../../../torch` or simply `/usr/torch`.
Note that in cmake, `Caffe2` is the name for the root `pytorch` project so `Caffe2_SOURCE_DIR` gives the `pytorch` directory; *not* the `caffe2` directory.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27449
Differential Revision: D17844763
Pulled By: ezyang
fbshipit-source-id: fcd964ef1b891972f18155eb72732e90f0d50b8b
