Merge pull request #13944 from sneaxiy/add_layer_norm_doc

Add layer_norm doc to illustrate param_attr and bias_attr

Author: sneaxiy

python/paddle/fluid/layers/nn.py

@@ -2317,19 +2317,28 @@ def layer_norm(input,
     Args:
         input(Variable): The input tensor variable.
         scale(bool): Whether to learn the adaptive gain :math:`g` after
-            normalization.
+            normalization. Default True.
         shift(bool): Whether to learn the adaptive bias :math:`b` after
-            normalization.
-        begin_norm_axis(bool): The normalization will be performed along
+            normalization. Default True.
+        begin_norm_axis(int): The normalization will be performed along
             dimensions from :attr:`begin_norm_axis` to :attr:`rank(input)`.
+            Default 1.
         epsilon(float): The small value added to the variance to prevent
-            division by zero.
+            division by zero. Default 1e-05.
         param_attr(ParamAttr|None): The parameter attribute for the learnable
-            gain :math:`g`.
+            gain :math:`g`. If :attr:`scale` is False, :attr:`param_attr` is
+            omitted. If :attr:`scale` is True and :attr:`param_attr` is None,
+            a default :code:`ParamAttr` would be added as scale. The 
+            :attr:`param_attr` is initialized as 1 if it is added. Default None. 
         bias_attr(ParamAttr|None): The parameter attribute for the learnable
-            bias :math:`b`.
+            bias :math:`b`. If :attr:`shift` is False, :attr:`bias_attr` is
+            omitted. If :attr:`shift` is True and :attr:`param_attr` is None,
+            a default :code:`ParamAttr` would be added as bias. The 
+            :attr:`bias_attr` is initialized as 0 if it is added. Default None.
         act(str): Activation to be applied to the output of layer normalizaiton.
-        name (str): The name of this layer. It is optional.
+                  Default None.
+        name(str): The name of this layer. It is optional. Default None, and a
+                   unique name would be generated automatically.
 
     Returns:
         ${y_comment}