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

Author: zhangbo9674

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