[reland][quant][graphmode][fx][refactor] Factor out insert_observers_for_model to a separate function (#54733) (#55307)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/55307
Test Plan: Imported from OSS
Reviewed By: vkuzo
Differential Revision: D27567475
fbshipit-source-id: 74b7db63f7e1e795e7ac7ed6027cf786d922e7bf
