[quant][fx][graphmode] Support standalone module in _convert_do_not_use (#70151)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70151
this supports converting an observed standalone module to quantized standalone module
in the new convert flow (convert observers to quant-dequant operators)
Test Plan:
```
python test/test_quant_trt.py TestConvertFxDoNotUse
```
Imported from OSS
Reviewed By: supriyar
Differential Revision: D33205163
fbshipit-source-id: 01ea44fb2a8ffe30bec1dd5678e7a72797bafafc
