* split some APIs from nn.py to rnn.py

* split some APIs from nn.py to sequence_lod.py

test=develop

* fix unit-test bug

test=develop

* fix test_layers unit-test bug

test=develop

* add_distributions_of_normal_and_uniform

* paddle/fluid/API.spec

* modify API.spec

* modified paddle/fluid/API.spec, test=develop

* modify paddle/fluid/API.spec, test=develop

* modify paddle/fluid/API.spec, test=develop

* fix some comment, test=develop

* modify API.spec, test=develop

* add comment for init function, modify hard code, test=develop

* modify API.spec, test=develop

* modify API.spec, test=develop

* make unit test function shorter, test=develop

* modify paddle/fluid/API.spec

* wip allreduce in op

* wip

* wip

* wip

* wip adding test

* wip for conflict with mp mode

* fix tests test=develop

* fix cpu build test=develop

* fix travis clang format test=develop

* fix cpu build test=develop

* update api.spec test=develop

* delete comment test=develop

* fix cpplint test=develop

* fix test=develop

* follow comment test=develop

* add file test=develop

* fix build test=develop

* update test=develop

* to be compatible with sync_bn, and fix mp mode in develop test=develop

* "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"

* change learning_rate_decay to learning_rate_scheduler
* optimize code
* change nn.cast to tensor.cast

* Implement multibox loss wrapper in Python API.

* Add some wrappers for SSD detection.

* Fix conflicts.

* Add unit testing for SSD loss wrapper.

* Update doc in Python API.

* Refine unit testing.

* Add more unit testing and update some interface arguments.

* "fix decode bug"

* "follow commnet"

* "fix error"

* "fix hook bug"

* fix based comment

* fix copyright

* fix based on comment

Users can use `a+b`, `a*10`.

* add copyright hook

* add copyright hook

* refine copyright hook

* "test copyright hook"

* fix check style

* fix ci

* Make cast op support bool

Also add `elemwise_sub/mul/abs/clip` layers

* Make fuild.layers as a module

* Move layers as a module

* Split layers.py into layers module

* Fix CI

* Fix CI