* dygraph support while_loop op
test=develop

* refine assign
test=develop

As the title

fix the doc in elementwise ops，add the override function in the doc

* support num_flatten_dims=-1 of API fc. test=develop

* fix name of class Test* and add CUDAPlace test. test=develop

Modify english document and unittest of while_loop

As the title.

* test=develop fix doc of detection's img_info param

* Add support for dynamic_decode(while) training. test=develop

* Fix assign_op and tensor_array_read_write_op after solving conflict. test=develop

* Fix test_rnn_decode_api.py. test=develop

* Refine docs for apis in rnn.py. test=develop

* Adjust outputs of dynamic_decode. test=develop

* Remove the force_cpu update in assign_op. test=develop

* Remove the force_cpu update in assign_op. test=develop

* Make RNNCell.get_initial_states support batch_dim_idx argument. test=develop

* Rename _create_array_outof_while as _create_array_out_of_while in rnn.py.
test=develop

This PR makes assign op support LoDTensorArray and enable the loop_vars in
while_loop to support tuple or list.

* fix circular dependent

* try import layers.nn from dygraph
test=develop

* Fixed warpctc, test=develop

* Set lod level of sequence_unpad's output to 1 in compile time

test=develop

* fix the en doc and example code of warpctc, test=develop, test=document_fix

* refine code, test=develop

* reuse contain_var, test=develop

* test=develop, bug fix for sparse recorder

* update layers, test=develop

* update layers for resnet, test=develop

* fix is_test attr, test=develop

* update cycle_gan, test=develop

* update reinforcement_learning, test=develop

* update ocr, test=develop

* fix bug, test=develop

* add example for reshape op

* modify fc to linear in sample code, test=develop

* remove FC, test=develop

* remove warnings, test=develop

* drop fluid/imperative/README.md , test=develop

* change fc to linear, test=develop

* polish code style, test=develop

1. Add a new input named batch_roi_nums for prroi_pool_op. batch_roi_nums includes the number of roi for each image in batch when rois is Tensor. This information is saved in rois's lod when rois is LoDTensor.
2. add grad check to prroi_pool_op and solve unnormal X grad diff in CPU.

* add erf op and python interface.

* add fp16 support for erf op.

* add unitests for erf op and its python interface.

* fix dim=[] bug in reduce ops

* append optimize op in the grad block of current block if current block is in control flow. test=develop

* add conditional grad op when optimizer used in control flow. test=develop

* add comment and modify typo. test=develop

* fix append_backward to support control flow. test=develop

* add test. test=develop

* fix copy_var_to_parent_block and conditional_block_grad. test=develop

* fix bug: revert to append conditional_block_grad vars to sub grad block. test=develop

* fix bug: revert to assign var to parent block even if var already is in parent block

* fix bug: consider outputs is empty. test=develop

* move _rename_grad_ out. test=develop

* modify code according to reviews from Huihuang. test=develop

* modify code according to reviews from Jinle. test=develop

* add shape description in accuracy api doc test=develop

* fix details test=develop

* fix details test=develop

* Fix example in log_loss doc, test=develop

* Use none for unknown dim, test=develop, test=document_fix

* convert dtype of vars for range op, test=develop

* update error information about batch_norm_grad

* update bn,test=develop

 fix the doc of scatter

* update layers used in mnist dygraph model, test=develop

* fix import issue, test=develop

* add dygraph utils, test=develop

* add unittest, test=develop

* fix stat shape back in global auc scenario
* add UT to cover global auc

* update layers, test=develop

* fix input numpy, test=develop

* fix bugs, test=develop

* follow commments, test=develop

* update getitem, test=develop

Update iou_similarity op to support non-normalized bbox

* add unittests, test=develop

* set dtype of compare op to bool, test=develop