a new section for ir generation (#81847)
This is to get a conversation started.
* @JackCaoG we could add attributes to items in `ir_codegen` section to customize IR generation logic (e.g. not generating `::Lower`). Though it could be a bit tricky to thread it through.
* Adding an extra argument to `map_codegen` to filter native functions out seems like a step in the right direction. Otherwise, it's a bit confusing how do we go from a full list to a codegen list.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81847
Approved by: https://github.com/JackCaoG, https://github.com/wconstab, https://github.com/bdhirsh
