1. set class GraphKernel as deprecated, and treat it as Cell
2. set class InplaceAssign as deprecated, suggested using Assign instead.
3. set op_selector as deprecated, removed the _selected_ops and _selected_grad_ops, replaced with real operations
4. removed the two passes of GraphKernel from frontend
5. removed the GraphKernel's codes from other modules

2.fix the backword of matrixinverse.

From: @yuan_shen_zhou
Reviewed-by: @liangchenghui,@wuxuejian
Signed-off-by: @liangchenghui

From: @TFbunny
Reviewed-by: @tom__chen,@tom__chen,@robingrosman
Signed-off-by: @robingrosman

A minor change in sigmoidGradGrad operation (non-functional change).

linting errors fix

comments cleanup

typo fix in doc

added more tests, fixed some formatting

changed expected dx values for linter

pylint const data fix

Merge pull request !4004 from fangzehua/squaresumall

Merge pull request !3846 from liuxiao93/Add-ops-SeluSquaredDifference

Merge pull request !3577 from fangzehua/many_grad_ops

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