Summary:
Removes aten/README.md (and some other files dating from when aten was its own repo), and moves the not outdated documentation into a note called "Tensor Basics". I updated the text lightly but did not overhaul the content.
CC zdevito
ezyang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/13601
Differential Revision: D12934480
Pulled By: goldsborough
fbshipit-source-id: 012a4267b4d6f27e4d5d55d6fc66363ddca10b41