* replace Backend by Place in C++ API

* fix left code

* fix test_to_api bug

* Refine to_tensor for eager mode

* support gpu_pinned

* refine code

* support gpu_pinned copy_to

* fix layer.__setattr__

* support to_tensor for gpu_pinned

* fix unit test

* refine gpu_pinned

* restore the original code

* add is_gup_pinned() and refine eager.Tensor._copy_to()

* refactor reducer

* modify cmakelists

* solve conflicts

* rename group and update process_group

* fix bugs of ProcessGroupNCCL

* modify for CIs

* refactoring reducer

* add processgroup_nccl