[quant][pt2] Improve prepare_qat Conv + BN numerics test (#100271)
Summary: This commit makes two improvements to the existing
test for Conv + BN fusion in `prepare_qat_pt2e`:
(1) Test `per_tensor_symmetric` in addition to `per_channel_symmetric`
(2) Initialize BN stats the same way in both flows. This is
necessary to get the `per_tensor_symmetric` case to pass.
Test Plan:
python test/test_quantization.py TestQuantizePT2E.test_prepare_qat_conv_bn_numerics
Reviewers: jerryzh168, kimishpatel
Differential Revision: [D45512851](https://our.internmc.facebook.com/intern/diff/D45512851)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100271
Approved by: https://github.com/jerryzh168
