[docs] Add 3D reduction example to tensordot docs (#45697)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45697
**Summary**
This commit adds an example of a reduction over three dimensions with
`torch.tensordot`. It is unclear from existing docs whether `dims`
should be a list of pairs or a pair of lists.
**Test Plan**
Built the docs locally.
*Before*
<img width="864" alt="Captura de Pantalla 2020-10-01 a la(s) 1 35 46 p m" src="https://user-images.githubusercontent.com/4392003/94866838-f0b17f80-03f4-11eb-8692-8f50fe3b9863.png">
*After*
<img width="831" alt="Captura de Pantalla 2020-10-05 a la(s) 12 06 28 p m" src="https://user-images.githubusercontent.com/4392003/95121092-670af600-0703-11eb-959f-73c7797a76ee.png">
**Fixes**
This commit closes #22748.
Test Plan: Imported from OSS
Reviewed By: ansley
Differential Revision: D24118186
Pulled By: SplitInfinity
fbshipit-source-id: c19b0b7e001f8cd099dc4c2e0e8ec39310510b46
