fx quant: clean up nit in insert_observer (#57367)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57367
This code is never hit (see insert_observer_for_output_of_the_node
which gates it out), so changing to an assert in order to
have `insert_observer` actually always insert an observer.
This helps code readability.
Test Plan:
```
python test/test_quantization.py TestQuantizeFx
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D28126898
fbshipit-source-id: 411bc37769a6eacbebc463ed6c84cac85871bd5e
