[quant][fx][graphmode][refactor] Change activation_post_process_map to track the observer name instead (#54643)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54643
A refactor needed for future changes.
Test Plan: Imported from OSS
Reviewed By: vkuzo
Differential Revision: D27314677
fbshipit-source-id: 972fbfb506f86da13f8817b3eaa5e6d0ad16ffe1
