[quant][fx][graphmode] Add backend_config_dict for standalone module (#70150)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70150
This PR allows user to specify backend_config_dict for standalone modules, both in prepare and convert step
adding this now to allow prototype for some of our customer use cases, test for the codepath will be added in
a separate PR
Test Plan:
regression tests
```
python test/test_quantization.py TestQuantizeFx
```
test that specifies backend_config for some module will be added in a separate PR for the use case we have in mind
since it requires other features
Imported from OSS
**Static Docs Preview: classyvision**
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Reviewed By: vkuzo
Differential Revision: D33205162
fbshipit-source-id: a657cef8e49d99b6a43653141521dc87c33bfd89
