Mention packed accessors in tensor basics doc (#19464)
Summary:
This is a continuation of efforts into packed accessor awareness.
A very simple example is added, along with the mention that the template can hold more arguments.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19464
Differential Revision: D15012564
Pulled By: soumith
fbshipit-source-id: a19ed536e016fae519b062d847cc58aef01b1b92
