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

The basic idea is: exploits data dependency to control the execution order
of side-effect operations, and keep the semantics of ANF unchanged.

The ControlDepend primitive is removed and there are two primitives added:

1. UpdateState:
```
  a = Assign(para, value)
```
became:
```
  a = Assign(para, value, u)
  u = UpdateState(u, a)
```

2. Load:
```
  x = Add(para, value)
```
became:
```
  p = Load(para, u)
  x = Add(p, value)
  u = UpdateState(u, p)
```

Signed-off-by: zhoufeng <zhoufeng54@huawei.com>

- 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.

Signed-off-by: zhoufeng <zhoufeng54@huawei.com>

Merge pull request !1914 from kisnwang/optimize-output-sync

Signed-off-by: zhoufeng <zhoufeng54@huawei.com>

Merge pull request !1359 from ZhangQinghua/master

1. Remove the records of user graphs;
2. Remove the records of user value nodes;
3. Remove the records of user cnodes;
4. Add the records of users, and the API to access the users of graph, value node, and cnode;
5. Fix issue:User cnode record may point to its own graph, when combine the user(caller) and used one(callee);
6. Fix issue:User graphs never update itself after its first creation.

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