add api to doc

doc/api/trainer_config_helpers/layers.rst

@@ -382,6 +382,15 @@ sampling_id_layer
     :members: sampling_id_layer
     :noindex:
 
+Slicing and Joining Layers
+==========================
+
+pad_layer
+-----------
+..  automodule:: paddle.trainer_config_helpers.layers
+    :members: pad_layer
+    :noindex:
+
 ..  _api_trainer_config_helpers_layers_cost_layers:
 
 Cost Layers

python/paddle/trainer_config_helpers/layers.py

@@ -108,6 +108,7 @@ __all__ = [
     'print_layer',
     'priorbox_layer',
     'spp_layer',
+    'pad_layer',
 ]