Port igamma and igammac to structured (#57626)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/57626
Test Plan: Imported from OSS
Reviewed By: VitalyFedyunin
Differential Revision: D28224834
Pulled By: ezyang
fbshipit-source-id: 924b0e10428429715dca1b75dce40dcd67b59134
