Merge pull request !2502 from geekun/master_fix_issue

Merge pull request !2241 from zhangzheng/accumulate

Merge pull request !2412 from jiangjinsheng/issue_fix4

- It provides a unified style to express graph and kernel for user.
- It provides a unified IR to represent graph and kernel for developer.
- It breaks the boundary between graph and kernel.
- It provides more opportunities to do compile optimization.

Merge pull request !2219 from jiangjinsheng/vm_splitv

Merge pull request !1962 from zhouneng/add_vm_support_for_ApproximateEqual_InplaceUpdateD_InTopKD

Merge pull request !1939 from lihongkang/lhk_master

Merge pull request !1926 from BowenK/op_elim

Merge pull request !1967 from zhangbuxue/check_arg_is_tensor_with_vm

Merge pull request !1862 from jiangjinsheng/issue_doc

Merge pull request !1884 from zhangbuxue/annotate_CellList_supports_methods

Merge pull request !1865 from JichenZhao/bnops_for_vm

* add infer_value for common math ops
* convert constant bool tensor to bool value

* do not infer value when encounter 0 as division

for while condition, do not unrool if condition is a tensor

Merge pull request !1770 from fangzehua/divnonan_vm_ops

Merge pull request !1750 from zhouneng/add_vm_support_for_expm1

Merge pull request !1528 from jiangjinsheng/vm_bessel

Merge pull request !1488 from fangzehua/fzh_edit

Merge pull request !1257 from candanzg/implicit_type_conversion2