[special] Add special.entr (#53500)
Summary:
Reference: https://github.com/pytorch/pytorch/issues/50345
TODO:
* [x] Verfiy docs rendering (https://11397990-65600975-gh.circle-artifacts.com/0/docs/special.html)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53500
Reviewed By: ngimel
Differential Revision: D27287096
Pulled By: mruberry
fbshipit-source-id: 6b3dfd53e811a0f023ee444a0b56176f825d39e9
