[docs][quant] Add fx graph mode quant api doc (#55306)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/55306
Test Plan: Imported from OSS
Reviewed By: vkuzo
Differential Revision: D27567187
fbshipit-source-id: ceef873b78fc77e366a47be66c8efd856bac013e
