[reland][quant][docs] Add fx graph mode quantization to quantization docs (#49211) (#49515)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/49515
Test Plan:
Imported from OSS
Imported from OSS
Reviewed By: vkuzo
Differential Revision: D25601061
fbshipit-source-id: 74e917d57895e9b4131a01fdcea8df3e94322bec
