[quant][graphmode][fx][refactor] Factor out insert_observers_for_model to a separate function (#54733)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/54733
Test Plan: Imported from OSS
Reviewed By: vkuzo
Differential Revision: D27408378
fbshipit-source-id: 9143f0a6f939fa80f1d1d6bae4b2d37aa21cb9b9
