[TensorExpr] Fast sigmoid for LLVM (#39717)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/39717
Test Plan: Imported from OSS
Reviewed By: ZolotukhinM
Differential Revision: D21949849
Pulled By: zheng-xq
fbshipit-source-id: f918bb2cb0ea647ce254fc51258af6fd01325f2d
