[Update transforms.py]use build-in `atanh` in TanhTransform (#40160)
Summary:
Since `torch.atanh` is recently implemented in https://github.com/pytorch/pytorch/issues/38388, we should simply use it for `TanhTransform`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40160
Differential Revision: D22208039
Pulled By: ezyang
fbshipit-source-id: 34dfbc91eb9383461e16d3452e3ebe295f39df26
