[quant][pt2] Add test for prepare_qat Conv + BN numerics (#99846)
Summary: This adds the test to compare the numerics of PT2 vs
FX after the Conv + BN fusion in prepare_qat.
Test Plan:
python test/test_quantization.py TestQuantizePT2E.test_prepare_qat_conv_bn_numerics
Reviewers: kimishpatel, jerryzh168
Differential Revision: [D45360706](https://our.internmc.facebook.com/intern/diff/D45360706)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99846
Approved by: https://github.com/jerryzh168
