adding quantization.rst file for quantization feature (#27559)
Summary:
This was written by Raghu, Jessica, Dmytro and myself.
This PR will accumulate additional changes (there are a few more things we need to add to this actual rst file). I'll probably add the related image files to this PR as well.
I'm breaking draft PR https://github.com/pytorch/pytorch/pull/27553 into more easily digestible pieces.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27559
Differential Revision: D17843414
Pulled By: gottbrath
fbshipit-source-id: 434689f255ac1449884acf81f10e0148d0d8d302
