[quant][pt2e] prepare_pt2e use quantization spec directly (#102054)
Summary:
In this PR we aligned with the design of annotation API and uses quantization spec directly for annotation.
main change is in prepare, we consume quantization_spec object directly instead of the observer or fake quant constructor, we create the constructor
inside prepare, and annotation api users only need to interact with quantization spec object after this PR
Test Plan:
```
buck2 test mode/opt caffe2/test:quantization_pt2e -- --exact 'caffe2/test:quantization_pt2e - test_resnet18_with_quantizer_api (quantization.pt2e.test_quantize_pt2e.TestQuantizePT2EModels)'
```
Reviewed By: kimishpatel
Differential Revision: D45934088
Pull Request resolved: https://github.com/pytorch/pytorch/pull/102054
Approved by: https://github.com/kimishpatel
