add import of ..common.layer_norm in __init__.py

Author: zhengqiwen1997

python/paddle/static/nn/__init__.py

@@ -33,6 +33,7 @@
 from ...tensor.creation import create_parameter  # noqa: F401
 from .loss import nce  # noqa: F401
 from .common import prelu  # noqa: F401
+from .common import layer_norm  # noqa: F401
 from ...fluid.layers import row_conv  # noqa: F401
 from ...fluid.layers import spectral_norm  # noqa: F401