quant docs: add API summary section (#45848)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45848
This is a resubmit of the following stack:
* start: https://github.com/pytorch/pytorch/pull/45093
* end: https://github.com/pytorch/pytorch/pull/45306
The original stack was reverted due to build failure,
resubmitting.
Test Plan: Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D24117781
Pulled By: vkuzo
fbshipit-source-id: fb767fff2b044cfbba695ca3949221904fc8931f
