Expose stats_mode N for NaiveSyncBN
Summary:
* Current code does not enable user to choose stats_mode="N" for naiveSyncBN. This change exposes that.
- stats_mode="N" is necessary for supporting zero-length inputs. Without it:  `SyncBatchNorm(stats_mode="") does not support zero batch size.`
@@ -142,6 +142,8 @@ def get_norm(norm, out_channels):
# for debugging:
"nnSyncBN": nn.SyncBatchNorm,
"naiveSyncBN": NaiveSyncBatchNorm,
+ # expose stats_mode N as an option to caller, required for zero-len inputs
+ "naiveSyncBN_N": lambda channels: NaiveSyncBatchNorm(channels, stats_mode="N"),
}[norm]
return norm(out_channels)
