* AutoPoly refactor to support integrating multi-backend targets easily
* Employ a pass/passmgr framework to manage all the transformations of ISL schedule tree in which transformation such as InitialSchTree and tileOuterBand would be considered as a pass to schedule tree.
* Refactor some data structure of poly so that they can de-couple with Davinci chips.
* Backend refactoring
* Enhance min alignment analysis with more accurate propagate conditions.
* Finetune pragma using alignment information before EmitInsn pass.
* Simplify EmitInsn pass by unifying the emit method for different patterns.
* change the way of using TVM
* Delete the repository ktvm and reference TVM directly in sourcecode(third_party/incubator-tvm).
* bring back the code which support GPU operator generation.
## Bugfixes
* fix wrong hoist problem in multicore loop switch hoist pass(!87).
* fix scalar rearrange bug(!84).
* fix matmul tuning and support all space tuning(!73).
* fix variable broadcast_idx redefinition error when pragma dma_copy is replaced by opt_broadcast(!45).
* fix the bug in broadcast_rewrite(!22).
* fix bugs of multi-core processing(!33).
* fix a bug that extra pipe_barrier inserted in the loop(!30).
* fix inefficient auto tiling for axis with tail and remove duplicated check(!6).
