[quant][pt2e][be] Cleanup prepare function in _pt2e (#103022)
Summary: att
Test Plan:
```
buck2 test mode/opt caffe2/test:quantization_pt2e -- 'caffe2/test:quantization_pt2e'
```
Differential Revision: D46346087
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103022
Approved by: https://github.com/andrewor14
