Remove unused ATen headers for mobile (#24850)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/24850
### Summary
There are 1373 header files in total that have been installed on mobile, many of which are not being used. Take ATen for example, there are 165 header files in total. Folders like `cuda/`, `cudann`, `miopen`, etc are not needed. This PR will remove 33 unnecessary header files as well as some cuda files.
### Test Plan
- `build_ios.sh` finished successfully
- `libtorch.a` can be compiled and run on mobile
Test Plan: Imported from OSS
Differential Revision: D16897314
Pulled By: xta0
fbshipit-source-id: 54e046936439a549fe633ec791a10a2a3d36fa8b
