* Fix relu and log

* Update nn.py

Fix log and relu layer

* Fix mean iou op.

* Fix crop layer test.

* Fix unitest

* fix unitest.

Fix and enhance beam_search_op and beam_searc_decode_op

* Modify lod_tensor.md and nn.py

* Modify control_flow.py doc

* undo change in lod_tensor.md

* Update some doc about layers' API.

* Fix format.

* Fix example bug in random_data_generator.

* Fix example bug in dropout.

* Follow comments and some small fix for some examples.