add index_sample (#2252)

doc/fluid/api/layers.rst

@@ -155,8 +155,8 @@ fluid.layers
     layers/image_resize.rst
     layers/image_resize_short.rst
     layers/increment.rst
-    layers/index_select.rst
     layers/index_sample.rst
+    layers/index_select.rst
     layers/inplace_abn.rst
     layers/instance_norm.rst
     layers/interpolate.rst

doc/fluid/api/layers/index_sample.rst

+..  THIS FILE IS GENERATED BY `gen_doc.{py|sh}`
+    !DO NOT EDIT THIS FILE MANUALLY!
+
+.. _api_fluid_layers_index_sample:
+
+index_sample
+------------
+
+..  autofunction:: paddle.fluid.layers.index_sample
+    :noindex:
+