[quant][graphmode][fx][refactor] Remove prepare_custom_config from Quantizer class (#59034)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59034
To remove Quantizer class and split prepare and convert functions to different files
Test Plan:
python test/test_quantization.py TestQuantizeFx
python test/test_quantization.py TestQuantizeFxOps
Imported from OSS
Reviewed By: vkuzo
Differential Revision: D28724873
fbshipit-source-id: 870e0822843ad1d035f41eaa015bdde9ccf6ec23
