[quant][graphmode][fx][refactor] Remove _convert from Quantizer class (#59042)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59042
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: D28724867
fbshipit-source-id: 9f87d51020caa20d5408cb2820947e23d92d5fc3
