Added factorial (#21129)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21129
ghimport-source-id: a676dd33c4d0b2b60c3e9ce725bda0abeb22375f
Differential Revision: D15563183
Pulled By: Chillee
fbshipit-source-id: 641cae34c181a16c772665f5f7ed01c96a67ea9c
