[quant][graphmode][fix] Handle the case when observed node has no users (#53210)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/53210
Test Plan: Imported from OSS
Reviewed By: vkuzo
Differential Revision: D26791724
fbshipit-source-id: b2a226a22d6aba86dd01cacbb56577048a289b3e
