add dataset rst file and add into gen_doc.sh (#749)

doc/fluid/api/dataset.rst

+..  THIS FILE IS GENERATED BY `gen_doc.{py|sh}`
+    !DO NOT EDIT THIS FILE MANUALLY!
+
+=============
+fluid.dataset
+=============
+
+.. _api_fluid_dataset_DatasetFactory:
+
+DatasetFactory
+--------------
+
+..  autoclass:: paddle.fluid.dataset.DatasetFactory
+    :members:
+    :noindex:
+
+.. _api_fluid_dataset_InMemoryDataset:
+
+InMemoryDataset
+---------------
+
+..  autoclass:: paddle.fluid.dataset.InMemoryDataset
+    :members:
+    :noindex:
+
+.. _api_fluid_dataset_QueueDataset:
+
+QueueDataset
+------------
+
+..  autoclass:: paddle.fluid.dataset.QueueDataset
+    :members:
+    :noindex:
+

doc/fluid/api/gen_doc.sh

 #!/bin/bash
 python gen_doc.py layers --submodules control_flow device io nn ops tensor learning_rate_scheduler detection metric_op > layers.rst
 
-for module in data_feeder clip metrics executor initializer io nets optimizer profiler regularizer transpiler recordio_writer backward average profiler
+for module in data_feeder dataset clip metrics executor initializer io nets optimizer profiler regularizer transpiler recordio_writer backward average profiler
 do
   python gen_doc.py ${module} > ${module}.rst
 done