[quant][graphmode][api] Add backend_config_dict to prepare_fx api (#64135)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64135
We want to start aligning the api with the design in https://github.com/pytorch/pytorch/wiki/Extending-PyTorch-Quantization-to-Custom-Backends
We plan to gradually move things from `prepare_custom_config_dict` and `convert_custom_config_dict`
to `backend_config_dict` and allow custom backend developer to define their own way of quantizing operators.
Test Plan:
python test/test_quantization.py TestQuantizeFx
python test/test_quantization.py TestQuantizeFxOps
Imported from OSS
Reviewed By: zou3519
Differential Revision: D30699456
fbshipit-source-id: e3c068da8d3da2270f57719f7159cc71cafa8598
