qat syncbn: add test coverage (#38738)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38738
Adds test coverage for swapping BN -> SyncBN on a fused ConvBN module.
Test Plan:
```
python test/test_quantization.py TestDistributed.test_qat_convbn_fused_syncbn_replacement
```
Imported from OSS
Differential Revision: D21648320
fbshipit-source-id: 3f7f71ec7b34d7d784dcbce9974c525b5db35942
