[quant][graphmode][fix] cloning schema in insert_observers (#40624)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40624
Previously we didn't clone schema, so the default schema is used, this is
causing issue for some models
Test Plan: Imported from OSS
Differential Revision: D22259519
fbshipit-source-id: e2a393a54cb18f55da0c7152a74ddc22079ac350
