* Add pad2d op.

* Add unitest and python api.

* Fix cuda op kernel.

* Fix python api.

* Fix python api.

* Update API.spec.

* Fix python api

* enhance pad_constant_like_op

* add API

* add API

This reverts commit f5d5d7b2.

* Disable in_place in batch_norm API.

* Fix scatter_op python API and remove inconsistency between implementation and doc

* API spec change

* Change as review comment

* Add three modes for prelu_op.

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 str and bytes problem
Fix sorted problem
Fix math problem
Fix CI problem

* fix the result of unit test

* fix resnext

* compare the result of PE and Exe

* compare the result of reduce and allreduce

* "remove inplace in single op"

* "fix ci"

* "add transpiler case"

* fix conflict

* "fix reshape"

* "delete reshape inplace attr"

* "follo the comments"

* "rerun ci"

* expose rank_loss python api

* Fix serious bug in nesterov momentum optimizer.

* remove block api

* remove clone_variable

* hide block inner apis

* update

* fix tests

* 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

This reverts commit bc28cf61.

* Fix mean iou op.

* Fix crop layer test.

* Fix unitest

* fix unitest.

* Add fill_zeros_array op. This op is used for zero-filling an LoDTensorArray.

* merge fill_zeros_array_op with fill_zeros_like_op

* add unit_test for fill_zeros_like for array

* bnorm+relu fuse for mkldnn

* separate fuse_relu function

* bug fix

* proper while range in inference_transpiler

* description fix

* review fix

* review fix

* unit test for fwd batch norm+relu MKLDNN fuse