Fix numerically instability of `SigmoidTransform` (#19802)
Summary:
fix #18254 for numerically instability of `SigmoidTransform`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19802
Differential Revision: D15701837
Pulled By: ezyang
fbshipit-source-id: fe6c755c523487c8bbdcc3bfb8455801617c70a4
