Clean extending autograd doc for output size 1 (#28860)
Summary:
Fix https://github.com/pytorch/pytorch/issues/28583
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28860
Differential Revision: D18224497
Pulled By: albanD
fbshipit-source-id: 0fa4eacce6f6092d555e509dc23bd75206f78d41
