quant docs: document how to customize qconfigs in eager mode (#45306)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45306
Adds details to the main quantization doc on how specifically
users can skip or customize quantization of layers.
Test Plan: Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D23917034
Pulled By: vkuzo
fbshipit-source-id: ccf71ce4300c1946b2ab63d1f35a07691fd7a2af
