* [Dygraph] Support sparse tensor in refactored reducer

* add uts

* refactor

* update

* fix bugs

* rename scalar_array to int_array

* update cmake

* fix conflict

* remove useless log

* replace Backend by Place in C++ API

* fix left code

* fix test_to_api bug

* fix bug

* fix bug

* refactor reducer

* modify cmakelists

* solve conflicts

* rename group and update process_group

* fix bugs of ProcessGroupNCCL

* modify for CIs

* refactoring reducer

* add reducer