[quant][graphmode][fx] Add support for weight and bias dtype in backend_config_dict (#68602)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68602
This PR adds support for configuring weight/bias dtype in backend_config_dict
and refactor the current code that checks when to insert observers
Test Plan:
```
python test/test_quantization.py TestQuantizeFx
python test/test_quantization.py TestQuantizeFxOps
```
Imported from OSS
Reviewed By: vkuzo
Differential Revision: D32537712
fbshipit-source-id: 28eb7c61a8dcad8c1f3f6622d490a34cff0c59e2
