add corner pooling (#2796)

* add center pooling

* fix yapf formatting

* add unittest

* fix flake8 formatting

* fix isort formatting

* fix isort formatting

* refactor and formatting

* remove top pool from unittext

* remove bottom pool from unittest

* fix bug in torch1.5

* add pytest for assertion

* center_pool -> corner_pool, add docstring

* fix unittest

* add corner_pool.py

* fix some weird logics

* move unittests of ops to a new folder

Co-authored-by: Kai Chen <chenkaidev@gmail.com>