override comparison operators in Python for Variable

Fix empty Vector foreach

Merge build docs with build using docker

rename switch_kernel.md to kernel_selection.md

* dynamic send/recv selected rows

* update by comment

* fix by comment

Fix #8368

reduce doc build time in travis ci

Improve split and concat op

* Fix constructor bug in mixed_vector

* Fix warnings

* Clean code

* Extract for-loop init. Make nvcc happy

Fix output dims of sequence expand op

* dist train support lr decay

* update by comment

* revert elementwise method creator

* delete comment

* Refine target_assign_op to unify the classification and regression targets assignment.

* Fix the unit testing.

* Fix conflicts.

test CI and update AUTHORS.md

* fix absolute include path

* Remove test_helper.h in old location

* update include path