Implement igammac (3rd PR) (#48171)
Summary:
Related: https://github.com/pytorch/pytorch/issues/46183 (torch.igamma)
This is the regularized upper incomplete gamma function.
This is supposed to be exactly the same as https://github.com/pytorch/pytorch/issues/47463, but after rebasing the `viable/strict` branch.
cc: mruberry
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48171
Reviewed By: zhangguanheng66
Differential Revision: D25060107
Pulled By: mruberry
fbshipit-source-id: 89780dea21dbb2141cbc4f7f18192cb78a769b17
