[quant][graphmode][fx][refactor] Remove _prepare from Quantizer class (#59038)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59038
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: D28724869
fbshipit-source-id: e8501c9720b5ddb654e78bc8fa08de0466c1d52b
