Add flatten op interface and enhance APIs about detection to support variable-length image. (#12422)

* add flatten api&enhance detection api

* unify shape_op data type

* update API.spec

* fix the result of unit test

* fix resnext

* compare the result of PE and Exe

* compare the result of reduce and allreduce

* api clean

* update API.spec

* "remove inplace in single op"

* "fix ci"

* "add transpiler case"

* fix conflict

* "fix reshape"

* "delete reshape inplace attr"

* "follo the comments"

* "rerun ci"

* Hidden APIs for While, StaticRNN, ParallelDo.

* Hiden some LoDTensor ralatted ops' Python wrapper.

* expose rank_loss python api

1. Make the feeding thread of py_reader a daemon thread.
2. Update buffer_reader's destructor, fixing a bug.
3. Make pyreader demo script supporting CPU environment.

* Fix serious bug in nesterov momentum optimizer.

fix AUC op
optimize it's test

* remove block api

* remove clone_variable

* hide block inner apis

* update

* fix tests

* fix auc

* update

* update

* fix compile

* fix param name

* add doc string

* fix test

* change im2sequence for ctc batch inference

* Update im2sequence_op.cc

* change im2sequence for ctc batch inference

* update

* change PR by comment

* fix ocr test error

* fix test_im2sequence

* modify the old name to standard name

* fix test_layers failed

* Add flag to set prior_box output order.

* Add region proposal network (RPN) target assign operator and Python API for Faster-RCNN.

* Add anchor generator operator for Faster-RCNN.
* Add unittest testing.
* Add Python API.