fx quant: move input_output_observed to qhandler (#57388)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57388
It's a bit confusing to have this be a decorator. It's simpler to
just expose it as a function on qhandler.
Test Plan:
```
python test/test_quantization.py TestQuantizeFxOps
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D28129411
fbshipit-source-id: f7316f285e8546c67e8d8cf753462b2c2abb2636
