fx quant: readability improvements on observer functions (#57368)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57368
1. renames functions which only sometimes insert observers to start with `maybe_`,
to clarify the difference from functions which always insert observers
2. saves a level of indent in `maybe_insert_observer_for_output_of_the_node`
Test Plan:
```
python test/test_quantization.py TestQuantizeFx
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D28126897
fbshipit-source-id: 4cbc184dbf5e85954314cfbbcdd1551474175bf0
