* Add pad2d op.

* Add unitest and python api.

* Fix cuda op kernel.

* Fix python api.

* Fix python api.

* Update API.spec.

* Fix python api

* Add unit test for rpn_target_assign API.

* Add generate_proposal_labels for Faster-RCNN.

* Add proposals generation operator for Faster-RCNN.

* Polish PrintOp

* Polish PrintOp

* Polish PrintOp

* Refine test_print_op

* enhance pad_constant_like_op

* add API

* add API

* refine dist train RPC deps

* clean up

* clean up

* fix ut

* remove input for fetch_barrier

* follow comments

* fix auc layer and add check for auc op

* use input to check if states are inited

* optimize code

This reverts commit f5d5d7b2.

* add is_test for while_op

* Change API

* 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

* Fix bug in conditional_block_op.
* Fix bug and add comments.
* Rename arguments.

Add dependency to send recv

* 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