[quant][graphmode][fx] Merge all quantization mode (#45292)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45292
This PR merges all quantization mode and will only expose the following top level functions:
```
prepare_fx
prepare_qat_fx
convert_fx
```
Test Plan: Imported from OSS
Reviewed By: vkuzo
Differential Revision: D23913105
fbshipit-source-id: 4e335286d6de225839daf51d1df54322d52d68e5
