SyncBN: preserve qconfig if it exists (#45317)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45317
Eager mode quantization depends on the presence of the `config`
model attribute. Currently converting a model to use `SyncBatchNorm`
removes the qconfig - fixing this. This is important if a BN is not
fused to anything during quantization convert.
Test Plan:
```
python test/test_quantization.py TestDistributed.test_syncbn_preserves_qconfig
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D23922072
fbshipit-source-id: cc1bc25c8e5243abb924c6889f78cf65a81be158
