[quant][pt2e][be] Add QuantizationSpecBase (#102582)
Summary:
Make all quantization spec to inherit from the same base class in order to simplify the typing
for QuantizationAnnotation
Test Plan:
```
buck2 test mode/opt caffe2/test:quantization_pt2e -- 'caffe2/test:quantization_pt2e'
```
Reviewed By: kimishpatel
Differential Revision: D46173954
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102582
Approved by: https://github.com/andrewor14
