Avoid CPU Sync in SyncBatchNorm When Capturing CUDA Graphs
We recently updated `SyncBatchNorm` to support empty input batches.
The new code removes stats from ranks with empty inputs. However,
this change breaks CUDA graph capture as it forces CPU sync. This
commit uses `is_current_stream_capturing()` to guard the new code
path, and only run the new code when not capturing CUA Graphs. To
support empty inputs with CUDA graph capturing, we might need to
update CUDA kernels for `batch_norm_backward_elemt` and
`batch_norm_gather_stats_with_counts`. See #78656.
Fixes #78549
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78666
Approved by: https://github.com/albanD
