* Fix relu and log

* Update nn.py

This reverts commit 4d8e8ee2, reversing
changes made to d6a9f005.

1. Add a new attribute named 'startuo_seed' to RandomCropOp. If the input
'Seed' is not initialized, the 'startup_seed' will be used to replace
it.

2. Refine CustomReader. Add a member variable 'scope_' to it. The
'scope_' will act as the global scope of preprocesing, making it
possiable to save something cross batches.

* "add some api reference"

* "fix based on preview"

* "fix evaluator"

* "remove template doc"

* "add some api reference"

* "fix based on preview"

* "fix evaluator"

* "remove template doc"

Give assign layer's second parameter 'output' a default value: None. If
it is None, the output variable will be created inside the layer.

* 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.