Update base for Update on "Remove unified.py (Quantizer and TwoStepQuantizer ABCs)"

Summary:
Delete `torchao/quantization/unified.py` which defined the `Quantizer`
and `TwoStepQuantizer` abstract base classes. These were trivial ABCs
that only declared method signatures (`quantize`, `prepare`, `convert`)
which all subclasses already implement. Remove the base class
inheritance from all subclasses and clean up imports.

Test Plan:
pytest test/quantization/test_qat.py -x
pytest test/quantization/test_quant_api.py -x

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Author: jerryzh168