* move fluid apis

* fix type error

* remove static exponential_decay

* fix some import error

* remove nn.py

* fix some error

* fix type error

* simple reaplce

* for debug

* fix bugs

* fix some bugs

* del fill_constant_batch_size_like

* remove api staticrnn

* move select_input/output to static/controw flow

* delete some func, only remain Switch

* clean fluid.layers.controw_flow

* remove fluid.layers.controlflow

* fix conditional_block ut

* remove fluid.layers.collective and related unittests.

* fix layers.init import

* fix all_gather method

* remove unused args in all_gather method

* remove loss.py

* fix __init__.py

* remove density_prior_box

* remove anchor_generator

* remove roi_perspective_transform

* remove generate_proposal_labels

* remove generate_mask_labels

* remove generate_proposals

* remove box_clip

* remove retinanet_detection_output

* remove multiclass_nms

* remove locality_aware_nms

* remove matrix_nms

* remove distribute_fpn_proposals

* remove box_decoder_and_assign

* remove collect_fpn_proposals

* remove 2 trt files

* move prior_box to static/nn/common.py

* move multi_box_head to static/nn/common.py

* fix for CI/CE

* remove retinanet_detection_output

* restore compile_vs_runtime_white_list.py

* restore test_retinanet_detection_output to white list

* replace nn.flatten by paddle.flatten, and fix doc for retinanet_target_assign

* add enable_static in demo and fix bug

* remove roi_perspective_transform in test_layers

* remove multi_box_head

* change self.multiclass_nms to _legacy_C_ops.multiclass_nms

* empty commit

* empty commit

* check code style

* fix prior_box

* fix CI

* remove redundant prior_box in detection.py

* fix docs

* remove detection

* fix prior_box en doc

* delete prior_box in common

* remote proir_box from __init__.py

* mv accuracy and auc

* rm distributions

* rm get_places

* replace metric

* remove apis in fluid.ops

* fix test_activation_nn_grad

* fix circle import error

* fix ops

* fix cos

* fix divide not inplace

* remove lazy-import part

* [CodeStyle] remove all future import

* revert test_error.py

* restore future import in example code

* Split some APIs from nn.py to loss.py

test=develop

* fix test_detection unit-test

test=develop

* 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