Re-enable C++ doc generation (#81719)
Reverts #80451, as this caused problems reported by many internal and external users. The generated C++ docs are used, even if they are lacking in human-generated content.
Fixes #80505
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81719
Approved by: https://github.com/kit1980, https://github.com/albanD
