fx quant prepare: clarify naming (#44125)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44125
In `Quantizer._prepare`, `observed` was used for two different variables
with different types. Making the names a bit cleaner and removing the
name conflict.
Test Plan:
```
python test/test_quantization.py TestQuantizeFx
python test/test_quantization.py TestQuantizeFxOps
```
Imported from OSS
Reviewed By: dskhudia
Differential Revision: D23504109
fbshipit-source-id: 0f73eac3d6dd5f72ad5574a4d47d33808a70174a
