Merge pull request !2241 from zhangzheng/accumulate

- 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 !1750 from zhouneng/add_vm_support_for_expm1

Merge pull request !1528 from jiangjinsheng/vm_bessel

Merge pull request !1558 from chenhaozhe/bert-optimiaztion

SparseApplyAdagrad example format

PReLU and grad state 1-d not supported

atan2 bprop dx dy revert to original shapes

Merge pull request !882 from zhangbuxue/fix_bprop_for_floor_mod

Signed-off-by: leonwanghui <leon.wanghui@huawei.com>