add api_doc to rst (#13279)

Author: chengduo

doc/fluid/api/layers.rst

@@ -822,6 +822,14 @@ pad
 ..  autofunction:: paddle.fluid.layers.pad
     :noindex:
 
+.. _api_fluid_layers_pad_constant_like:
+
+pad_constant_like
+---
+
+..  autofunction:: paddle.fluid.layers.pad_constant_like
+    :noindex:
+
 .. _api_fluid_layers_label_smooth:
 
 label_smooth
@@ -1145,6 +1153,14 @@ sigmoid
 ..  autofunction:: paddle.fluid.layers.sigmoid
     :noindex:
 
+.. _api_fluid_layers_hsigmoid:
+
+hsigmoid
+-------
+
+..  autofunction:: paddle.fluid.layers.hsigmoid
+    :noindex:
+
 .. _api_fluid_layers_logsigmoid:
 
 logsigmoid