Add SoftplusTransform (#52300)
Summary:
This pull request introduces `SoftplusTransform` to `torch.distributions.transforms`. `SoftplusTransform` transforms via the mapping `Softplus(x) = log(1 + exp(x))`. Note that the transform is different to [`torch.nn.Softplus`](https://pytorch.org/docs/stable/generated/torch.nn.Softplus.html#torch.nn.Softplus), as that has additional `beta` and `threshold` parameters. Inverse and `log_abs_det_jacobian` for a more complex `SoftplusTransform` can be added in the future.
vitkl fritzo
Addresses the issue discussed here: [pyro issue 855](https://github.com/pyro-ppl/numpyro/issues/855)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52300
Reviewed By: albanD, ejguan
Differential Revision: D34082655
Pulled By: neerajprad
fbshipit-source-id: 6114e74ee5d73c1527191bed612a142d691e2094
(cherry picked from commit a181a3a9e53a34214a503d38760ad7778d08a680)
