fix sync_bn error in fp16 amp-o2 (#40943)

python/paddle/fluid/dygraph/amp/auto_cast.py

@@ -171,7 +171,7 @@ def pure_fp16_initialize(models):
             if (layer._dtype == 'float16') or isinstance(
                     layer, (paddle.nn.BatchNorm, paddle.nn.BatchNorm1D,
                             paddle.nn.BatchNorm2D, paddle.nn.BatchNorm3D,
-                            paddle.nn.LayerNorm)):
+                            paddle.nn.LayerNorm, paddle.nn.SyncBatchNorm)):
                 continue
             layer._to_impl(dtype='float16', include_sublayers=False)
     return models