[quant][pt2e][fix] Fix the internal test failures caused by refactor (#98378)
Summary: att, this PR removes some incorrect assumptions from `_maybe_insert_observers_before_graph_output`
Test Plan:
internal test
Differential Revision: D44697212
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98378
Approved by: https://github.com/andrewor14
