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.`

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Reviewed By: jonmorton, vaibhava0, ppwwyyxx

Differential Revision: D29362934

fbshipit-source-id: f8d5a33cac599f706e6620c31495e8c1577970dc

Author: Carolyn Soo

detectron2/layers/batch_norm.py

@@ -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)