[quant][pt2] Add Conv + BN fusion for prepare QAT (#98568)
**Summary:** This commit adds the `prepare_qat_pt2e` API and the
fusion logic for Conv + BN. We use the subgraph rewriter to
match and replace the pattern with the existing logic in
`nniqat.ConvBn2d`. Note this is not the end-to-end flow yet.
In particular, the convert flow needs to swap the new subgraph
with another one that merges the batchnorm stats back into conv.
The Conv + BN fusion is implemented in the following steps:
1. Annotate all nodes in the pattern `[conv - bn - getitem]`
2. Match and replace this pattern with the fused QAT pattern
(note that this is a larger subgraph than the original one)
3. Copy over metadata from the original nodes to the
corresponding nodes in the new subgraph, to ensure the
stack traces and dtype annotations are preserved
4. Prepare will insert fake quantizes in the right places
based on the annotations
**Test Plan:**
python test/test_quantization.py TestQuantizePT2E.test_qat_conv_bn_fusion
**Reviewers:** jerryzh168, kimishpatel, yanboliang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98568
Approved by: https://github.com/kimishpatel
