[quant][pt2] Add prepare QAT test for resnet18 (#103020)
Summary:
Prepare QAT for resnet18 has matching numerics with FX.
Adding this test requires us to refactor the way the test code
is structured, however.
Test Plan: python test/test_quantization.py TestQuantizePT2EModels.test_qat_resnet18
Differential Revision: D46456243
Pull Request resolved: https://github.com/pytorch/pytorch/pull/103020
Approved by: https://github.com/kimishpatel
